Thermodynamic simulation method is developed and applied to analyze the performance and the NOx emission characteristics of the IGCC (Integrated Gasification Combined Cycle) power plants coupled with ASU (Air Separation Unit). Simulations on IGCC power plants are made through combining the chemical process models for coal gasification and gas clean-up and the thermodynamic combined cycle model with NOx prediction capability. With coal as feedstock of IGCC, the present study investigates and compares the power output, the overall efficiency and the NOx emission characteristics of various IGCC plants at different ASU integration conditions in order to give the design criteria for efficient and environmental friendly IGCC configuration.