<PAGE>

            Securities and Exchange Commission on February 2, 2000

================================================================================
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549
                            ______________________
                           POST-EFFECTIVE AMENDMENT
                             NO. 1 TO FORM S-4 ON
                                   FORM S-8*

                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933
                            ______________________
                                  DYNEGY INC.
            (Exact name of Registrant as specified in its charter)

             Illinois                                    74-2928353
    (State or other jurisdiction                       (I.R.S. Employer
  of incorporation or organization)                 Identification Number)
  1000 Louisiana Street, Suite 5800                        77002
           Houston, Texas                               (Zip Code)
(Address of Principal Executive Offices)

         Dynegy Inc. Amended and Restated Employee Equity Option Plan
            Dynegy Inc. Amended and Restated 1991 Stock Option Plan
        Illinova Corporation 1992 Long-Term Incentive Compensation Plan
                           (Full title of the plan)


                              Kenneth E. Randolph
                       1000 Louisiana Street, Suite 5800
                             Houston, Texas 77002
                                (713) 507-6400
                  (Name and address, including zip code, and
                    telephone number of agent for service)

                                With Copies to:

                               Julien R. Smythe
                   Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                       1900 Pennzoil Place - South Tower
                                 711 Louisiana
                             Houston, Texas 77002
                                (713) 220-5800

                            ______________________
                        CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
                Title of Securities                    Amount to be   Proposed Maximum    Proposed Maximum      Amount of
                  to be Registered                    Registered (1)   Offering Price    Aggregate Offering   Registration
                                                                          Per Share             Price              Fee
--------------------------------------------------------------------------------------------------------------------------
<S>                                                   <C>             <C>                <C>                  <C>
Class A Common Stock, no par value..................   10,500,000 (2)        (3)                  (3)              (3)
==========================================================================================================================
</TABLE>

(1)  The number of shares of Class A common stock registered hereby is subject
     to adjustment to prevent dilution resulting from stock splits, stock
     dividends or similar transactions.
(2)  Consisting of 10,500,000 shares of Class A common stock of the registrant
     reserved for issuance under the Dynegy Inc. Amended Employee Equity Option
     Plan, the Dynegy Inc. Amended and Restated 1991 Stock Option Plan and the
     Illinova Corporation 1992 Long-Term Incentive Plan. At the effective time
     of the merger of Dynegy Inc., since renamed Dynegy Holdings Inc., a
     Delaware corporation ("Old Dynegy"), and Illinova corporation, an Illinois
     corporation ("Illinova"), the Old Dynegy plans and the Illinova plan were
     assumed by Dynegy and each share of common stock of Illinova issuable under
     the Illinova plan was converted into an equal number of shares of Class A
     common stock of Dynegy. Each share of common stock of Old Dynegy issuable
     under the Old Dynegy plans was converted into 0.69 shares of Class A common
     stock of Dynegy.
(3)  Not applicable.  All filing fees payable in connection with the issuance of
     these securities were paid in connection with the filing of Dynegy's
     Registration Statement on Form S-4 (File Number 333-84965)  filed September
     7, 1999.

* Filed as a Post-effective Amendment to such Form S-4 registration statement
  pursuant to the procedure described herein in the section captioned
  "Introductory Statement."
                             ______________________

================================================================================

<PAGE>

                             INTRODUCTORY STATEMENT

     On February 1, 2000, Illinova Corporation, an Illinois corporation
("Illinova"), and Dynegy Inc., a Delaware corporation (since renamed Dynegy
Holdings Inc.) ("Old Dynegy"), each merged with subsidiaries of Energy
Convergence Holding Company, an Illinois corporation (since renamed Dynegy Inc.)
("Dynegy") pursuant to the Agreement and Plan of Merger dated as of June 14,
1999, as amended, by and among Illinova, Old Dynegy, Energy Convergence
Acquisition Corporation, Dynegy Acquisition Corporation and Dynegy. Pursuant to
Section 12 g-3 of the Securities Exchange Act of 1934, Dynegy is the successor
to Old Dynegy. Old Dynegy common stock, par value $0.01 per share is no longer
transferable, and certificates evidencing shares of Old Dynegy common stock
represent only the right to receive, without interest, 0.69 shares of Dynegy
Class A common stock, no par value in accordance with the provisions of the
merger agreement. Illinova common stock, no par value, is no longer
transferable, and certificates evidencing shares of Illinova common stock
represent only the right to receive, without interest, one share of Dynegy Class
A common stock in accordance with the provisions of the merger agreement.
Pursuant to the merger agreement, Dynegy assumed the stock option plans of Old
Dynegy and Illinova, and each unexpired and unexercised outstanding option to
purchase Old Dynegy common stock was automatically converted into an option to
purchase that number of shares of Dynegy Class A common stock obtained by
multiplying the number of shares of Old Dynegy common stock issuable upon
exercise of such option by 0.69 at an exercise price per share equal to the per
share exercise price of such Old Dynegy stock option divided by 0.69. Each
unexpired and unexercised outstanding option to purchase Illinova common stock
was automatically converted into an option to purchase the equivalent number of
shares of Dynegy Class A common stock at the same exercise price per share.

     Dynegy hereby amends its registration statement on Form S-4 (File Number
333-84965) by filing this Post-effective Amendment No. 1 to the Registration
Statement on Form S-4 on this Form S-8 relating to up to 10,500,000 shares of
Dynegy Class A common stock, issuable under the Dynegy Employee Equity Option
Plan, the Dynegy Amended and Restated 1991 Stock Option Plan, and the Illinova
1992 Long-Term Incentive Compensation Plan. All such shares of Dynegy Class A
common stock were previously registered on the Form S-4, and are being
transferred to this Form S-8.


                                    PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

     The documents containing the information specified in Part I of Form S-8
will be sent or given to participants as specified by Rule 428(b)(1) promulgated
under the Securities Act of 1933, as amended (the "Securities Act").

                                       1

<PAGE>

                                    PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.
         ---------------------------------------

      The following documents filed by Dynegy or its predecessor with the
Securities and Exchange Commission (the "Commission") are incorporated by
reference in this registration statement:

      (a)  Dynegy's Annual Report on Form 10-K for the fiscal year ended
           December 31, 1998;

      (b)  Dynegy's Quarterly Report on Form 10-Q for the quarterly period
           ended March 31, 1999;

      (c)  Dynegy's Proxy Statement relating to the Annual Meeting of
           Shareholders of Dynegy held on May 21, 1999 (other than the
           compensation committee report and stock performance chart);

      (d)  Dynegy's Current Report on Form 8-K filed with the Commission on June
           14, 1999 relating to the execution of the merger agreement and
           related documents;

      (e)  Dynegy's Quarterly Report on Form 10-Q for the quarterly period ended
           June 30, 1999;

      (f)  Dynegy's Current Report on Form 8-K filed with the Commission on July
           27, 1999 announcing a senior note offering by Dynegy;

      (g)  Dynegy's Current Report on Form 8-K filed with the Commission on July
           30, 1999 announcing the filing of a T-1;

      (h)  Dynegy's Current Report on Form 8-K filed with the Commission on
           August 4, 1999 relating to Dynegy's second quarter earnings;

      (i)  Dynegy's Quarterly Report on Form 10-Q for the quarterly period ended
           September 30, 1999;

      (j)  Dynegy's Current Report on Form 8-K filed with the Commission on
           January 6, 2000 relating to an advertisement in the New York Times
           regarding the merger closing;

      (k)  Dynegy's Registration Statement of Dynegy Class A common stock on
           Form 8-A filed with the Commission on February 1, 2000;

      (l)  Dynegy's Current Report on Form 8-K filed with the Commission on
           February 2, 2000 regarding the closing of the merger; and

      (m)  The description of the Class A common stock and the Dynegy Inc. 2000
           Long-Term Incentive Plan contained in Dynegy's Registration Statement
           on Form S-4 (File Number 333-84965) declared effective by the
           Commission on September 8, 1999.

      In addition, all documents subsequently filed by Dynegy pursuant to
Sections 13(a), 13(c), 14 and 15(d) of the Securities Exchange Act of 1934, as
amended, prior to the filing of a post-effective amendment to this registration
statement that indicates that all securities offered hereby have been sold or
that deregisters all securities then remaining unsold, shall be deemed to be
incorporated by reference in this registration statement and to be a part hereof
from the date of filing of such documents.  Any statement contained in a
document incorporated or deemed to be incorporated by reference herein shall be
deemed to be modified or superseded for purposes of this registration statement
to the extent that a statement contained herein modifies or supersedes such
statement.

Item 4.  Description of Securities.
         -------------------------

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.
         --------------------------------------

      Not applicable.

                                       2

<PAGE>

Item 6.  Indemnification of Directors and Officers.
         -----------------------------------------

      Illinois law permits corporations to adopt a provision in their charters
eliminating the liability of a director to the corporation or its shareholders
for monetary damages for breach of the director's fiduciary duty of care.

      Illinois corporations are not permitted to eliminate monetary liability
where such liability is based on:

       (1)  any breach of the director's duty of loyalty to the corporation or
            its shareholders;

       (2)  acts or omissions not in good faith or that involve intentional
            misconduct or a knowing violation of law;

       (3)  liability of directors for unlawful payment of dividends or unlawful
            stock purchases or redemptions; or

       (4)  any transaction from which the director derived an improper personal
            benefit.

     The Dynegy articles of incorporation also provide for broad mandatory
indemnification, generally extending to any person who is or was a director or
officer who acted in good faith and in a manner he reasonably believed to be not
opposed to the best interests of the corporation. Such articles also eliminate
the liability of directors to the fullest extent permissible under Illinois law,
as such law exists currently or as it may be amended in the future. The Dynegy
bylaws provide that Dynegy may indemnify an employee or agent of the corporation
in a proceeding, including those by or in the right of the corporation, because
he or she is or was acting in the scope of his or her duties, in good faith, and
in a manner he or she reasonably believed to be in the best interests of the
corporation.

     The merger agreement provides that Dynegy will indemnify the present and
former directors and officers of Old Dynegy, Illinova and their respective
subsidiaries and fiduciaries under their benefit plans against all losses,
damages, costs or expenses (including attorneys' fees), liabilities or amounts
paid in settlement arising out of actual or alleged actions or omissions
existing at or before the effective date of the merger. The indemnification will
be to the fullest extent permitted under Illinois law or Dynegy's articles of
incorporation and bylaws in effect on the date of such indemnification and Old
Dynegy's and Illinova's indemnification obligations in effect as of the date of
the merger agreement. Dynegy will also be obligated to advance expenses as
incurred to the fullest extent permitted by law. The determination of compliance
with the indemnification requirements will be made by independent counsel
mutually acceptable to Dynegy and the indemnified person.

     For six years after the effective date of the merger, Dynegy will maintain
Illinova's and Old Dynegy's existing directors' and officers' liability
insurance policies covering the existing directors and officers to the extent
related to liabilities prior to the merger. The coverage will be at least as
favorable as the existing coverage, but Dynegy will not be required to spend
over 150% of the higher of the annual premium paid by Old Dynegy or Illinova for
its existing coverage.

Item 7.  Exemption from Registration Claimed.
         -----------------------------------

      Not applicable.

Item 8.  Exhibits.
         --------

     3.1  -  Articles of Incorporation of Dynegy (incorporated by reference from
             the exhibits to the Registration Statement of Dynegy on Form S-4,
             File Number 333-84965, filed with the Commission on September 7,
             1999).

     3.2  -  Articles of Amendment to Dynegy's Articles of Incorporation dated
             August 27, 1999 (incorporated by reference from the exhibits to the
             Registration Statement of Dynegy on Form S-4, File Number
             333-84965, filed with the Commission on September 7, 1999).

                                       3

<PAGE>

     3.3   -  Articles of Amendment to Dynegy's Articles of Incorporation dated
              January 4, 2000 (incorporated by reference from the exhibits to
              Dynegy's Form 8-A, File Number 1-11156, filed with the Commission
              on February 1, 2000).

     3.4   -  Statement of Resolution Establishing Series of Series A
              Convertible Preferred Stock of Dynegy Inc. filed with the Illinois
              Secretary of State on February 1, 2000 (incorporated by reference
              from the exhibits to Dynegy's Form 8-A, File Number 1-11156, filed
              with the Commission on February 1, 2000).

     3.5   -  Bylaws of Dynegy (incorporated by reference from the exhibits to
              the Registration Statement of Dynegy on Form S-4, File Number
              333-84965, filed with the Commission on September 7, 1999).

     4.1   -  Dynegy Employee Equity Option Plan (incorporated by reference from
              the exhibits to the Registration Statement of Trident-NGL Holding,
              Inc. on Form S-4, File Number 33-88907).

     4.2   -  Dynegy Amended and Restated 1991 Stock Option Plan (incorporated
              by reference from the exhibits to the Registration Statement of
              Midstream Combination Corp. on Form S-4, File Number 333-09419).

     4.3   -  Illinova 1992 Long-Term Incentive Compensation Plan (incorporated
              by reference from the exhibits to the Registration Statement of
              Illinois Power Company on Form S-8, as amended, File Number
              33-60124).

     5.1*  -  Opinion of Troutman Sanders LLP as to the legality of the shares
              being registered.

     23.1* -  Consent of Troutman Sanders LLP (included in the opinion filed as
              Exhibit 5.1 to this Registration Statement).

     23.2* -  Consent of Arthur Andersen LLP.

     24.1* -  Powers of Attorney (included in the signature page of this Post-
              effective Amendment No. 1).
     ___________________________
     *filed herewith

Item 9.  Undertakings.
         ------------

      (a)    The undersigned registrant hereby undertakes:

             (1)    To file, during any period in which offers or sales are
being made, a post-effective amendment to this registration statement:

             (i)    To include any prospectus required by Section 10(a)(3) of
the Securities Act;

             (ii)   To reflect in the prospectus any facts or events arising
after the effective date of the registration statement (or the most recent post-
effective amendment thereof) which, individually or in the aggregate, represent
a fundamental change in the information set forth in the registration statement.
Notwithstanding the foregoing, any increase or decrease in volume of securities
offered (if the total dollar value of securities offered would not exceed that
which was registered) and any deviation from the low or high end of the
estimated maximum offering range may be reflected in the form of prospectus
filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the
changes in volume and price represent no more than 20 percent change in the
maximum aggregate offering price set forth in the "Calculation of Registration
Fee" table in the effective registration statement.

             (iii)  To include any material information with respect to the plan
of distribution not previously disclosed in the registration statement or any
material change to such information in the registration statement.


                                       4
<PAGE>

             Provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) do not
apply if the registration statement is on Form S-3, Form S-8 or Form F-3 and the
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed with or furnished to the
Commission by the registrant pursuant to Section 13 or Section 15(d) of the
Exchange Act that are incorporated by reference in the registration statement.

             (2)    That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

             (3)    To remove from registration by means of a post-effective
amendment any of the securities being registered which remain unsold at the
termination of the offering.

      (b)    The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act) that is
incorporated by reference in the registration statement shall be deemed to be a
new registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

      (c)    Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the registrant pursuant to the foregoing provisions, or otherwise, the
registrant has been advised that in the opinion of the Commission such
indemnification is against public policy as expressed in the Securities Act and
is, therefore, unenforceable. In the event that a claim for indemnification
against such liabilities (other than the payment by the registrant of expenses
incurred or paid by a director, officer or controlling person of the registrant
in the successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Securities Act and will be governed by the final
adjudication of such issue.

                                       5

<PAGE>

                                   SIGNATURES


      Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this Registration
Statement on Form S-8 to be signed on its behalf by the undersigned, thereunto
duly authorized, in the City of Houston, State of Texas, on February 2, 2000.

                          DYNEGY INC.



                          By: /s/ C. L. Watson
                              ----------------
                              C. L. Watson
                              Chairman of the Board and Chief Executive Officer

      The undersigned directors and officers of Dynegy hereby constitute and
appoint C. L. Watson and Kenneth E. Randolph with full power to act without the
other and with full power of substitution, our true and lawful attorneys-in-fact
with full power to execute in our name and behalf in the capacities indicated
below any and all amendments (including post-effective amendments and amendments
thereto) to this Registration Statement on Form S-8 and to file the same, with
all exhibits thereto and other documents in connection therewith with the
Commission and hereby ratify and confirm all that such attorneys-in-fact, or
either of them, or their substitutes shall lawfully do or cause to be done by
virtue hereof.

      Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement on Form S-8 has been signed below by the following
persons in the capacities indicated on February 2, 2000.

Signature                                   Title
---------                                   -----

/s/ C. L. Watson                            Chairman of the Board, Director
-------------------------                   and Chief Executive Officer
C. L. Watson                                (Principal Executive Officer)

/s/ John U. Clarke                          Executive Vice President and
-------------------------                   Chief Financial Officer
John U. Clarke                              (Principal Financial Officer)

/s/ Bradley P. Farnsworth                   Senior Vice President and Controller
-------------------------                   (Principal Accounting Officer)
Bradley P. Farnsworth

/s/ Charles E. Bayless                      Director
-------------------------
Charles E. Bayless

/s/ Stephen W. Bergstrom                    Director
-------------------------
Stephen W. Bergstrom



<PAGE>

  Signature                             Title
  ---------                             -----

/s/ J. Otis Winters                     Director
------------------------
J. Otis Winters

/s/ Daniel L. Dienstbier                Director
------------------------
Daniel L. Dienstbier

/s/ Patricia A. Woertz                  Director
------------------------
Patricia A. Woertz

/s/ Darald W. Callahan                  Director
------------------------
Darald W. Callahan

/s/ Richard H. Matzke                   Director
------------------------
Richard H. Matzke

/s/ J. Joe Adorjan                      Director
------------------------
J. Joe Adorjan

/s/ C. Steven McMillan                  Director
------------------------
C. Steven McMillan

/s/ Robert M. Powers                    Director
------------------------
Robert M. Powers

/s/ Sheli Z. Rosenberg                  Director
------------------------
Sheli Z. Rosenberg

/s/ Joe J. Stewart                      Director
------------------------
Joe J. Stewart

/s/ John D. Zeglis                      Director
------------------------
John D. Zeglis


<PAGE>

                               INDEX TO EXHIBITS


Exhibit
Number   Description of Exhibits
------   -----------------------

 3.1     Articles of Incorporation of Dynegy (incorporated by reference from the
         exhibits to the Registration Statement of Dynegy on Form S-4, File
         Number 333-84965, filed with the Commission on September 7, 1999).

 3.2     Articles of Amendment to Dynegy's Articles of Incorporation dated
         August 27, 1999 (incorporated by reference from the exhibits to the
         Registration Statement of Dynegy on Form S-4, File Number 333-84965,
         filed with the Commission on September 7, 1999).

 3.3     Articles of Amendment to Dynegy's Articles of Incorporation dated
         January 4, 2000 (incorporated by reference from the exhibits to
         Dynegy's Form 8-A, File Number 1-11156, filed with the Commission on
         February 1, 2000).

3.4      Statement of Resolution Establishing Series of Series A Convertible
         Preferred Stock of Dynegy Inc. filed with the Illinois Secretary of
         State on February 1, 2000 (incorporated by reference from the exhibits
         to Dynegy's Form 8-A, File Number 1-11156, filed with the Commission on
         February 1, 2000).

3.5      Bylaws of Dynegy (incorporated by reference from the exhibits to the
         Registration Statement of Dynegy on Form S-4, File Number 333-84965,
         filed with the Commission on September 7, 1999).

4.1      Dynegy Employee Equity Option Plan (incorporated by reference from the
         exhibits to the Registration Statement of Trident-NGL Holding, Inc. on
         Form S-4, File Number 33-88907).

4.2      Dynegy Amended and Restated 1991 Stock Option Plan (incorporated by
         reference from the exhibits to the Registration Statement of Midstream
         Combination Corp. on Form S-4, File Number 333-09419).

4.3      Illinova 1992 Long-Term Incentive Compensation Plan (incorporated by
         reference from the exhibits to the Registration Statement of Illinois
         Power Company on Form S-8, as amended, File Number 33-60124).

5.1*     Opinion of Troutman Sanders LLP as to the legality of the shares being
         registered.

23.1*    Consent of Troutman Sanders LLP (included in the opinion filed as
         Exhibit 5.1 to this Registration Statement).

23.2*    Consent of Arthur Andersen LLP.

24.1*    Powers of Attorney (included in the signature page of this Post-
         effective Amendment No. 1).

--------------------

*filed herewith
[{"type": "S-8", "sequence": "1", "description": "POST-EFFECTIVE AMDT NO. 1 TO FORM S-4 ON FORM S-8", "secsgml_size_bytes": 27813}]
{"company-data": {"conformed-name": "DYNEGY INC /IL/", "cik": "0000879215", "assigned-sic": "1311", "irs-number": "742928353", "state-of-incorporation": "IL", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-96045", "film-number": "520744"}, "business-address": {"street1": "1000 LOUISIANA", "street2": "STE 5800", "city": "HOUSTON", "state": "TX", "zip": "62521", "phone": "7133677600"}, "mail-address": {"street1": "1000 LOUISIANA", "street2": "SUITE 5800", "city": "HOUSTON", "state": "TX", "zip": "62521"}, "former-company": [{"former-conformed-name": "DYNEGY INC", "date-changed": "19980717"}, {"former-conformed-name": "TRIDENT NGL HOLDING INC", "date-changed": "19930916"}]}
Securities and Exchange Commission on February 2, 2000

================================================================================
                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549
                            ______________________
                           POST-EFFECTIVE AMENDMENT
                             NO. 1 TO FORM S-4 ON
                                   FORM S-8*

                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933
                            ______________________
                                  DYNEGY INC.
            (Exact name of Registrant as specified in its charter)

             Illinois                                    74-2928353
    (State or other jurisdiction                       (I.R.S. Employer
  of incorporation or organization)                 Identification Number)
  1000 Louisiana Street, Suite 5800                        77002
           Houston, Texas                               (Zip Code)
(Address of Principal Executive Offices)

         Dynegy Inc. Amended and Restated Employee Equity Option Plan
            Dynegy Inc. Amended and Restated 1991 Stock Option Plan
        Illinova Corporation 1992 Long-Term Incentive Compensation Plan
                           (Full title of the plan)

                              Kenneth E. Randolph
                       1000 Louisiana Street, Suite 5800
                             Houston, Texas 77002
                                (713) 507-6400
                  (Name and address, including zip code, and
                    telephone number of agent for service)

                                With Copies to:

                               Julien R. Smythe
                   Akin, Gump, Strauss, Hauer & Feld, L.L.P.
                       1900 Pennzoil Place - South Tower
                                 711 Louisiana
                             Houston, Texas 77002
                                (713) 220-5800

                            ______________________
                        CALCULATION OF REGISTRATION FEE
Title of Securities                    Amount to be   Proposed Maximum    Proposed Maximum      Amount of
                  to be Registered                    Registered (1)   Offering Price    Aggregate Offering   Registration
                                                                          Per Share             Price              Fee
--------------------------------------------------------------------------------------------------------------------------
Class A Common Stock, no par value..................   10,500,000 (2)        (3)                  (3)              (3)
==========================================================================================================================
(1)  The number of shares of Class A common stock registered hereby is subject
     to adjustment to prevent dilution resulting from stock splits, stock
     dividends or similar transactions.
(2)  Consisting of 10,500,000 shares of Class A common stock of the registrant
     reserved for issuance under the Dynegy Inc. Amended Employee Equity Option
     Plan, the Dynegy Inc. Amended and Restated 1991 Stock Option Plan and the
     Illinova Corporation 1992 Long-Term Incentive Plan. At the effective time
     of the merger of Dynegy Inc., since renamed Dynegy Holdings Inc., a
     Delaware corporation ("Old Dynegy"), and Illinova corporation, an Illinois
     corporation ("Illinova"), the Old Dynegy plans and the Illinova plan were
     assumed by Dynegy and each share of common stock of Illinova issuable under
     the Illinova plan was converted into an equal number of shares of Class A
     common stock of Dynegy. Each share of common stock of Old Dynegy issuable
     under the Old Dynegy plans was converted into 0.69 shares of Class A common
     stock of Dynegy.
(3)  Not applicable.  All filing fees payable in connection with the issuance of
     these securities were paid in connection with the filing of Dynegy's
     Registration Statement on Form S-4 (File Number 333-84965)  filed September
     7, 1999.

* Filed as a Post-effective Amendment to such Form S-4 registration statement
  pursuant to the procedure described herein in the section captioned
  "Introductory Statement."
                             ______________________

================================================================================
INTRODUCTORY STATEMENT

     On February 1, 2000, Illinova Corporation, an Illinois corporation
("Illinova"), and Dynegy Inc., a Delaware corporation (since renamed Dynegy
Holdings Inc.) ("Old Dynegy"), each merged with subsidiaries of Energy
Convergence Holding Company, an Illinois corporation (since renamed Dynegy Inc.)
("Dynegy") pursuant to the Agreement and Plan of Merger dated as of June 14,
1999, as amended, by and among Illinova, Old Dynegy, Energy Convergence
Acquisition Corporation, Dynegy Acquisition Corporation and Dynegy. Pursuant to
Section 12 g-3 of the Securities Exchange Act of 1934, Dynegy is the successor
to Old Dynegy. Old Dynegy common stock, par value $0.01 per share is no longer
transferable, and certificates evidencing shares of Old Dynegy common stock
represent only the right to receive, without interest, 0.69 shares of Dynegy
Class A common stock, no par value in accordance with the provisions of the
merger agreement. Illinova common stock, no par value, is no longer
transferable, and certificates evidencing shares of Illinova common stock
represent only the right to receive, without interest, one share of Dynegy Class
A common stock in accordance with the provisions of the merger agreement.
Pursuant to the merger agreement, Dynegy assumed the stock option plans of Old
Dynegy and Illinova, and each unexpired and unexercised outstanding option to
purchase Old Dynegy common stock was automatically converted into an option to
purchase that number of shares of Dynegy Class A common stock obtained by
multiplying the number of shares of Old Dynegy common stock issuable upon
exercise of such option by 0.69 at an exercise price per share equal to the per
share exercise price of such Old Dynegy stock option divided by 0.69. Each
unexpired and unexercised outstanding option to purchase Illinova common stock
was automatically converted into an option to purchase the equivalent number of
shares of Dynegy Class A common stock at the same exercise price per share.

     Dynegy hereby amends its registration statement on Form S-4 (File Number
333-84965) by filing this Post-effective Amendment No. 1 to the Registration
Statement on Form S-4 on this Form S-8 relating to up to 10,500,000 shares of
Dynegy Class A common stock, issuable under the Dynegy Employee Equity Option
Plan, the Dynegy Amended and Restated 1991 Stock Option Plan, and the Illinova
1992 Long-Term Incentive Compensation Plan. All such shares of Dynegy Class A
common stock were previously registered on the Form S-4, and are being
transferred to this Form S-8.

                                    PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

     The documents containing the information specified in Part I of Form S-8
will be sent or given to participants as specified by Rule 428(b)(1) promulgated
under the Securities Act of 1933, as amended (the "Securities Act").

                                       1
PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.
         ---------------------------------------

      The following documents filed by Dynegy or its predecessor with the
Securities and Exchange Commission (the "Commission") are incorporated by
reference in this registration statement:

      (a)  Dynegy's Annual Report on Form 10-K for the fiscal year ended
           December 31, 1998;

      (b)  Dynegy's Quarterly Report on Form 10-Q for the quarterly period
           ended March 31, 1999;

      (c)  Dynegy's Proxy Statement relating to the Annual Meeting of
           Shareholders of Dynegy held on May 21, 1999 (other than the
           compensation committee report and stock performance chart);

      (d)  Dynegy's Current Report on Form 8-K filed with the Commission on June
           14, 1999 relating to the execution of the merger agreement and
           related documents;

      (e)  Dynegy's Quarterly Report on Form 10-Q for the quarterly period ended
           June 30, 1999;

      (f)  Dynegy's Current Report on Form 8-K filed with the Commission on July
           27, 1999 announcing a senior note offering by Dynegy;

      (g)  Dynegy's Current Report on Form 8-K filed with the Commission on July
           30, 1999 announcing the filing of a T-1;

      (h)  Dynegy's Current Report on Form 8-K filed with the Commission on
           August 4, 1999 relating to Dynegy's second quarter earnings;

      (i)  Dynegy's Quarterly Report on Form 10-Q for the quarterly period ended
           September 30, 1999;

      (j)  Dynegy's Current Report on Form 8-K filed with the Commission on
           January 6, 2000 relating to an advertisement in the New York Times
           regarding the merger closing;

      (k)  Dynegy's Registration Statement of Dynegy Class A common stock on
           Form 8-A filed with the Commission on February 1, 2000;

      (l)  Dynegy's Current Report on Form 8-K filed with the Commission on
           February 2, 2000 regarding the closing of the merger; and

      (m)  The description of the Class A common stock and the Dynegy Inc. 2000
           Long-Term Incentive Plan contained in Dynegy's Registration Statement
           on Form S-4 (File Number 333-84965) declared effective by the
           Commission on September 8, 1999.

      In addition, all documents subsequently filed by Dynegy pursuant to
Sections 13(a), 13(c), 14 and 15(d) of the Securities Exchange Act of 1934, as
amended, prior to the filing of a post-effective amendment to this registration
statement that indicates that all securities offered hereby have been sold or
that deregisters all securities then remaining unsold, shall be deemed to be
incorporated by reference in this registration statement and to be a part hereof
from the date of filing of such documents.  Any statement contained in a
document incorporated or deemed to be incorporated by reference herein shall be
deemed to be modified or superseded for purposes of this registration statement
to the extent that a statement contained herein modifies or supersedes such
statement.

Item 4.  Description of Securities.
         -------------------------

      Not applicable.

Item 5.  Interests of Named Experts and Counsel.
         --------------------------------------

      Not applicable.

                                       2
Item 6.  Indemnification of Directors and Officers.
         -----------------------------------------

      Illinois law permits corporations to adopt a provision in their charters
eliminating the liability of a director to the corporation or its shareholders
for monetary damages for breach of the director's fiduciary duty of care.

      Illinois corporations are not permitted to eliminate monetary liability
where such liability is based on:

       (1)  any breach of the director's duty of loyalty to the corporation or
            its shareholders;

       (2)  acts or omissions not in good faith or that involve intentional
            misconduct or a knowing violation of law;

       (3)  liability of directors for unlawful payment of dividends or unlawful
            stock purchases or redemptions; or

       (4)  any transaction from which the director derived an improper personal
            benefit.

     The Dynegy articles of incorporation also provide for broad mandatory
indemnification, generally extending to any person who is or was a director or
officer who acted in good faith and in a manner he reasonably believed to be not
opposed to the best interests of the corporation. Such articles also eliminate
the liability of directors to the fullest extent permissible under Illinois law,
as such law exists currently or as it may be amended in the future. The Dynegy
bylaws provide that Dynegy may indemnify an employee or agent of the corporation
in a proceeding, including those by or in the right of the corporation, because
he or she is or was acting in the scope of his or her duties, in good faith, and
in a manner he or she reasonably believed to be in the best interests of the
corporation.

     The merger agreement provides that Dynegy will indemnify the present and
former directors and officers of Old Dynegy, Illinova and their respective
subsidiaries and fiduciaries under their benefit plans against all losses,
damages, costs or expenses (including attorneys' fees), liabilities or amounts
paid in settlement arising out of actual or alleged actions or omissions
existing at or before the effective date of the merger. The indemnification will
be to the fullest extent permitted under Illinois law or Dynegy's articles of
incorporation and bylaws in effect on the date of such indemnification and Old
Dynegy's and Illinova's indemnification obligations in effect as of the date of
the merger agreement. Dynegy will also be obligated to advance expenses as
incurred to the fullest extent permitted by law. The determination of compliance
with the indemnification requirements will be made by independent counsel
mutually acceptable to Dynegy and the indemnified person.

     For six years after the effective date of the merger, Dynegy will maintain
Illinova's and Old Dynegy's existing directors' and officers' liability
insurance policies covering the existing directors and officers to the extent
related to liabilities prior to the merger. The coverage will be at least as
favorable as the existing coverage, but Dynegy will not be required to spend
over 150% of the higher of the annual premium paid by Old Dynegy or Illinova for
its existing coverage.

Item 7.  Exemption from Registration Claimed.
         -----------------------------------

      Not applicable.

Item 8.  Exhibits.
         --------

     3.1  -  Articles of Incorporation of Dynegy (incorporated by reference from
             the exhibits to the Registration Statement of Dynegy on Form S-4,
             File Number 333-84965, filed with the Commission on September 7,
             1999).

     3.2  -  Articles of Amendment to Dynegy's Articles of Incorporation dated
             August 27, 1999 (incorporated by reference from the exhibits to the
             Registration Statement of Dynegy on Form S-4, File Number
             333-84965, filed with the Commission on September 7, 1999).

                                       3
3.3   -  Articles of Amendment to Dynegy's Articles of Incorporation dated
              January 4, 2000 (incorporated by reference from the exhibits to
              Dynegy's Form 8-A, File Number 1-11156, filed with the Commission
              on February 1, 2000).

     3.4   -  Statement of Resolution Establishing Series of Series A
              Convertible Preferred Stock of Dynegy Inc. filed with the Illinois
              Secretary of State on February 1, 2000 (incorporated by reference
              from the exhibits to Dynegy's Form 8-A, File Number 1-11156, filed
              with the Commission on February 1, 2000).

     3.5   -  Bylaws of Dynegy (incorporated by reference from the exhibits to
              the Registration Statement of Dynegy on Form S-4, File Number
              333-84965, filed with the Commission on September 7, 1999).

     4.1   -  Dynegy Employee Equity Option Plan (incorporated by reference from
              the exhibits to the Registration Statement of Trident-NGL Holding,
              Inc. on Form S-4, File Number 33-88907).

     4.2   -  Dynegy Amended and Restated 1991 Stock Option Plan (incorporated
              by reference from the exhibits to the Registration Statement of
              Midstream Combination Corp. on Form S-4, File Number 333-09419).

     4.3   -  Illinova 1992 Long-Term Incentive Compensation Plan (incorporated
              by reference from the exhibits to the Registration Statement of
              Illinois Power Company on Form S-8, as amended, File Number
              33-60124).

     5.1*  -  Opinion of Troutman Sanders LLP as to the legality of the shares
              being registered.

     23.1* -  Consent of Troutman Sanders LLP (included in the opinion filed as
              Exhibit 5.1 to this Registration Statement).

     23.2* -  Consent of Arthur Andersen LLP.

     24.1* -  Powers of Attorney (included in the signature page of this Post-
              effective Amendment No. 1).
     ___________________________
     *filed herewith

Item 9.  Undertakings.
         ------------

      (a)    The undersigned registrant hereby undertakes:

             (1)    To file, during any period in which offers or sales are
being made, a post-effective amendment to this registration statement:

             (i)    To include any prospectus required by Section 10(a)(3) of
the Securities Act;

             (ii)   To reflect in the prospectus any facts or events arising
after the effective date of the registration statement (or the most recent post-
effective amendment thereof) which, individually or in the aggregate, represent
a fundamental change in the information set forth in the registration statement.
Notwithstanding the foregoing, any increase or decrease in volume of securities
offered (if the total dollar value of securities offered would not exceed that
which was registered) and any deviation from the low or high end of the
estimated maximum offering range may be reflected in the form of prospectus
filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the
changes in volume and price represent no more than 20 percent change in the
maximum aggregate offering price set forth in the "Calculation of Registration
Fee" table in the effective registration statement.

             (iii)  To include any material information with respect to the plan
of distribution not previously disclosed in the registration statement or any
material change to such information in the registration statement.

                                       4
Provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) do not
apply if the registration statement is on Form S-3, Form S-8 or Form F-3 and the
information required to be included in a post-effective amendment by those
paragraphs is contained in periodic reports filed with or furnished to the
Commission by the registrant pursuant to Section 13 or Section 15(d) of the
Exchange Act that are incorporated by reference in the registration statement.

             (2)    That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

             (3)    To remove from registration by means of a post-effective
amendment any of the securities being registered which remain unsold at the
termination of the offering.

      (b)    The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act (and, where applicable, each filing of an employee benefit plan's
annual report pursuant to Section 15(d) of the Exchange Act) that is
incorporated by reference in the registration statement shall be deemed to be a
new registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

      (c)    Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the registrant pursuant to the foregoing provisions, or otherwise, the
registrant has been advised that in the opinion of the Commission such
indemnification is against public policy as expressed in the Securities Act and
is, therefore, unenforceable. In the event that a claim for indemnification
against such liabilities (other than the payment by the registrant of expenses
incurred or paid by a director, officer or controlling person of the registrant
in the successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Securities Act and will be governed by the final
adjudication of such issue.

                                       5
SIGNATURES

      Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this Registration
Statement on Form S-8 to be signed on its behalf by the undersigned, thereunto
duly authorized, in the City of Houston, State of Texas, on February 2, 2000.

                          DYNEGY INC.

                          By: /s/ C. L. Watson
                              ----------------
                              C. L. Watson
                              Chairman of the Board and Chief Executive Officer

      The undersigned directors and officers of Dynegy hereby constitute and
appoint C. L. Watson and Kenneth E. Randolph with full power to act without the
other and with full power of substitution, our true and lawful attorneys-in-fact
with full power to execute in our name and behalf in the capacities indicated
below any and all amendments (including post-effective amendments and amendments
thereto) to this Registration Statement on Form S-8 and to file the same, with
all exhibits thereto and other documents in connection therewith with the
Commission and hereby ratify and confirm all that such attorneys-in-fact, or
either of them, or their substitutes shall lawfully do or cause to be done by
virtue hereof.

      Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement on Form S-8 has been signed below by the following
persons in the capacities indicated on February 2, 2000.

Signature                                   Title
---------                                   -----

/s/ C. L. Watson                            Chairman of the Board, Director
-------------------------                   and Chief Executive Officer
C. L. Watson                                (Principal Executive Officer)

/s/ John U. Clarke                          Executive Vice President and
-------------------------                   Chief Financial Officer
John U. Clarke                              (Principal Financial Officer)

/s/ Bradley P. Farnsworth                   Senior Vice President and Controller
-------------------------                   (Principal Accounting Officer)
Bradley P. Farnsworth

/s/ Charles E. Bayless                      Director
-------------------------
Charles E. Bayless

/s/ Stephen W. Bergstrom                    Director
-------------------------
Stephen W. Bergstrom
Signature                             Title
  ---------                             -----

/s/ J. Otis Winters                     Director
------------------------
J. Otis Winters

/s/ Daniel L. Dienstbier                Director
------------------------
Daniel L. Dienstbier

/s/ Patricia A. Woertz                  Director
------------------------
Patricia A. Woertz

/s/ Darald W. Callahan                  Director
------------------------
Darald W. Callahan

/s/ Richard H. Matzke                   Director
------------------------
Richard H. Matzke

/s/ J. Joe Adorjan                      Director
------------------------
J. Joe Adorjan

/s/ C. Steven McMillan                  Director
------------------------
C. Steven McMillan

/s/ Robert M. Powers                    Director
------------------------
Robert M. Powers

/s/ Sheli Z. Rosenberg                  Director
------------------------
Sheli Z. Rosenberg

/s/ Joe J. Stewart                      Director
------------------------
Joe J. Stewart

/s/ John D. Zeglis                      Director
------------------------
John D. Zeglis
INDEX TO EXHIBITS

Exhibit
Number   Description of Exhibits
------   -----------------------

 3.1     Articles of Incorporation of Dynegy (incorporated by reference from the
         exhibits to the Registration Statement of Dynegy on Form S-4, File
         Number 333-84965, filed with the Commission on September 7, 1999).

 3.2     Articles of Amendment to Dynegy's Articles of Incorporation dated
         August 27, 1999 (incorporated by reference from the exhibits to the
         Registration Statement of Dynegy on Form S-4, File Number 333-84965,
         filed with the Commission on September 7, 1999).

 3.3     Articles of Amendment to Dynegy's Articles of Incorporation dated
         January 4, 2000 (incorporated by reference from the exhibits to
         Dynegy's Form 8-A, File Number 1-11156, filed with the Commission on
         February 1, 2000).

3.4      Statement of Resolution Establishing Series of Series A Convertible
         Preferred Stock of Dynegy Inc. filed with the Illinois Secretary of
         State on February 1, 2000 (incorporated by reference from the exhibits
         to Dynegy's Form 8-A, File Number 1-11156, filed with the Commission on
         February 1, 2000).

3.5      Bylaws of Dynegy (incorporated by reference from the exhibits to the
         Registration Statement of Dynegy on Form S-4, File Number 333-84965,
         filed with the Commission on September 7, 1999).

4.1      Dynegy Employee Equity Option Plan (incorporated by reference from the
         exhibits to the Registration Statement of Trident-NGL Holding, Inc. on
         Form S-4, File Number 33-88907).

4.2      Dynegy Amended and Restated 1991 Stock Option Plan (incorporated by
         reference from the exhibits to the Registration Statement of Midstream
         Combination Corp. on Form S-4, File Number 333-09419).

4.3      Illinova 1992 Long-Term Incentive Compensation Plan (incorporated by
         reference from the exhibits to the Registration Statement of Illinois
         Power Company on Form S-8, as amended, File Number 33-60124).

5.1*     Opinion of Troutman Sanders LLP as to the legality of the shares being
         registered.

23.1*    Consent of Troutman Sanders LLP (included in the opinion filed as
         Exhibit 5.1 to this Registration Statement).

23.2*    Consent of Arthur Andersen LLP.

24.1*    Powers of Attorney (included in the signature page of this Post-
         effective Amendment No. 1).

--------------------

*filed herewith