A computer simulation model in ASPEN PLUS shell has been developed to simulate the performance of IGCC and IGHAT cycle power plants. The model was used to study the effects of design and performance parameters on the efficiency and emissions from IGCC and IGHAT cycles. The simulation models are capable of performing mass, energy and exergy balances which may be used to trace system inefficiencies to their source component thereby providing insights into component interactions within the cycles and act as pointers to system optimization trade-offs.