Coal gasification is the core unit of the integrated gasification combined cycle (IGCC) system, and decoupling the exergy destruction caused by internal irreversibility during its energy conversion is of great significance for further optimization of the gasification process. In this study, two exergy analysis models, the equilibrium model and the kinetic model, are developed to characterize coal gasification with O2-H2O. Exergy analysis combined with gasification kinetics is performed to decouple the reaction-induced exergy destruction, and the role of different reactions is revealed. Results indicate that gasification reactions are the primary source of exergy destruction, accounting for 49.7% of the total exergy destruction under the base conditions, with char-steam gasification (R3) causing the largest destruction, followed by the combustion reactions of CH4 (R6) and char (R1). Corresponding exergy destruction mechanisms are elucidated by the energy utilization diagram (EUD) analysis and the comparison of reaction extent of different gasification reactions. On this basis, four feasible control methods are proposed to reduce the gasification exergy destruction. Moreover, the effects of the O2-to-coal mass ratio  and H2O-to-coal mass ratio on gasification performance are discussed, and the optimal exergy efficiency boundaries are identified. Corresponding kinetic analysis reveals the change mechanism of exergy destruction along different boundaries. It is found that the optimal exergy efficiency at different gasification temperatures is located on the carbon deposition boundary, and the water-gas shift reaction (R7) can serve as an indicator of the coupling degree of gasification reactions that reflects the magnitude of reaction-induced exergy destruction.