<PAGE>


                       WAIVER, RELEASE AND SUBSTITUTION
                       --------------------------------

     This WAIVER, RELEASE AND SUBSTITUTION  made as of February 17, 1997, by 
and between Destec Energy, Inc., a Delaware corporation ("Destec"), The Dow 
Chemical Company, a Delaware corporation ("TDCC") and NGC Corporation, a 
Delaware corporation ("NGC").

                                  RECITALS

     A.   Destec entered into certain Compensation Adjustment Agreements 
dated November 1, 1991 (the "Compensation Adjustment Agreements");

     B.   TDCC gave Destec a Guaranty, dated December 18, 1991 (the 
"Guaranty"), whereby TDCC agreed to guarantee the performance of Destec of 
its covenants and agreements under the Compensation Adjustment Agreements;

     C.   TDCC has entered into an Agreement and Plan of Merger, dated 
February 17, 1997, with NGC, Destec and NGC Acquisition Corporation II (the 
"Acquisition Agreement") whereby NGC has agreed to acquire TDCC's interest in 
Destec (the "Acquisition"); 

     D.   In connection with the Acquisition, TDCC desires to be released from 
all of its obligations under or in connection with the Guaranty; 

     E.   In connection with the Acquisition Agreement, NGC has agreed to be 
substituted in TDCC's stead, to assume all of TDCC's obligations under or in 
connection with the Guaranty and to indemnify TDCC for any and all liability 
which may arise under or in connection with said Guaranty;

     F.   In accordance with the terms and conditions of this Waiver, Release 
And Substitution, Destec consents to the substitution of NGC in TDCC's stead 
and the assumption by NGC of all of TDCC's obligations under or in connection 
with the Guaranty; and 

     G.   In accordance with the terms and conditions of the Guaranty, Destec 
and NGC give the waiver and release set forth in this Waiver, Release And 
Substitution.

     NOW, THEREFORE, in consideration of the premises and the respective 
undertakings of the parties set forth below and other good and valuable 
consideration, the receipt and sufficiency of which are acknowledged, it is 
agreed as follows:


1.   SUBSTITUTION

     Effective as of the Effective Time as that term is defined in the 
Acquisition Agreement, NGC is hereby substituted in TDCC's stead and agrees 
to assume all of TDCC's rights and obligations under or in connection with 
the Guaranty.

<PAGE>

2.   INDEMNIFICATION

     As of the Effective Time, NGC on its own behalf and on behalf of its 
successors and assigns, hereby agrees to indemnify and hold harmless TDCC, 
its successors and assigns, and each of its officers, shareholders, 
directors, partners, agents and employees, and their respective successors 
and assigns (each "Indemnified Party" and collectively the "Indemnified 
Parties"), and shall not seek contribution from TDCC, or any of the 
Indemnified Parties, for any cost, damage, loss, liability, obligation, 
penalty, claim, action, suit or expense, including attorneys' fees, imposed, 
asserted or incurred in connection with any and all claims, rights, demands, 
actions, suits, causes of action, damages, counterclaims, defenses, losses, 
costs, obligations, liabilities and expenses of every kind or nature, known 
or unknown, suspected or unsuspected, fixed or contingent, foreseen or 
unforeseen, liquidated or unliquidated, whether arising heretofore or 
hereafter, based upon any actual or alleged fiduciary or other duty or 
obligation to Destec, the individuals with whom Destec entered into 
Compensation Adjustment Agreements, or either of their respective successors 
or assigns, whether based upon common law, statute, or otherwise, in 
connection with the obligations set forth in the Guaranty.

3.   CONSENT TO SUBSTITUTION

     As of the Effective Time, Destec on its own behalf and on behalf of its 
successors and assigns, hereby consents to the substitution of NGC in TDCC's 
stead and the assumption by NGC of all of TDCC's obligations, under or in 
connection with the Guaranty.   After the date this Agreement is signed, 
Destec and NGC will use reasonable best efforts to obtain the written consent 
of the Destec employees with Compensation Adjustment Agreements still covered 
by the Guaranty as of the date of this Agreement, which individuals are 
listed on Schedule A.

4.   WAIVER AND RELEASE

     As of the Effective Time, Destec and NGC on their own behalf and on 
behalf of their successors and assigns, hereby release and discharge TDCC, 
its successors and assigns, and each of its officers, shareholders, 
directors, partners, agents and employees, and their respective successors 
and assigns (each a "Released Party" and collectively the "Released 
Parties"), from any and all claims, rights, demands, actions, suits, causes 
of action, damages, counterclaims, defenses, losses, costs, obligations, 
liabilities and expenses of every kind or nature, including attorney's fees, 
known or unknown, suspected or unsuspected, fixed or contingent, foreseen or 
unforeseen, liquidated or unliquidated, whether arising heretofore or 
hereafter, based upon any actual or alleged fiduciary or other duty or 
obligation to Destec, the individuals with whom Destec entered into 
Compensation Adjustment Agreements, or either of their respective successors 
or assigns, whether based upon common law, statute, or otherwise, in 
connection with the obligations set forth in the Guaranty;

5.   MISCELLANEOUS

<PAGE>

     A.   Amendment and Modification: Any amendment or modification made to 
this Waiver, Release And Substitution must be so made in a writing signed by 
all the parties.

     B.   Successors and Assigns: This Waiver, Release And Substitution and 
the provisions hereof shall inure to the benefit of and be binding upon the 
successors and assigns of Destec, NGC and TDCC. 

     C.   No Third-Party Beneficiaries: Except as otherwise expressly 
provided nothing in this Waiver, Release And Substitution, expressed or 
implied, is intended to confer any rights or remedies upon any person or 
entity, other than the parties hereto, the Indemnified Parties and the 
Released Parties.

     D.   Counterparts: This Waiver, Release And Substitution may be executed 
in one or more counterparts, all of which together shall constitute one 
agreement binding on all parties to this Waiver, Release And Substitution.

     E.   Applicable Law: This Waiver, Release And Substitution shall be 
governed by and construed in accordance with the laws of the State of 
Delaware without giving effect to the principles of conflicts of law thereof.

     IN WITNESS WHEREOF, the undersigned, intending to be bound
hereby, have duly executed this Waiver, Release And Substitution
as of the day and year first above written.

                               DESTEC ENERGY, INC.,
                               a Delaware corporation

                               By:  /s/ Enrique M. Larroucau
                                   -----------------------------------------
                               Name:   Enrique M. Larroucau
                               Title:  Senior Vice President, Chief
                                       Financial Officer and Treasurer

                               THE DOW CHEMICAL COMPANY, 
                               a Delaware Corporation 

                               By:  /s/ B.G. Taylorson
                                   -----------------------------------------
                               Name:   B.G. Taylorson
                               Title:  Corporate Director, Mergers &
                                       Acquisitions

                               NGC CORPORATION
                               a Delaware corporation

                               By:  /s/ Kenneth E. Randolph
                                   -----------------------------------------
                               Name:   Kenneth E. Randolph
                               Title:  Senior Vice President and
                                       General Counsel
<PAGE>

                                  SCHEDULE A

          P.A. Agnew                               J.J. Stauffacher

          B.L. Audish                              K.W. Moser

          F.E. Bozeman                             D.K. Livingstone

          B.E. Ybarguen                            E.P. Hermann

          M.D. Thomas                              L.D. Garman

          D.G. Sundstrom                           C. Goff
<PAGE>

     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        P.A. Agnew

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        B.L. Audish

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        F.E. Bozeman

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        B.E. Ybarguen

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.


                                     By: 
                                        --------------------------------------
                                        M.D. Thomas

                                     Date:
<PAGE>

     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        D.G. Sundstrom

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                       J.J. Stauffacher

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                         K.W. Moser

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        D.K. Livingstone

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        E.P. Hermann

<PAGE>


                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        L.D. Garman

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        E.L. Spencer

                                     Date:


     IN WITNESS WHEREOF, the undersigned, consents to the Waiver, Release And 
Substitution by and between Destec Energy, Inc., The Dow Chemical Company and 
NGC Corporation made as of February 17, 1997.

                                     By: 
                                        --------------------------------------
                                        C.F. Goff

                                     Date: