Integrated Gasification Combined Cycle (IGCC) power system has attractive prospective well into the next century with its high efficiency and excellent environmental superiority. At present, many demonstration power plants have been built in the world. Research in this area is also actively carried on in China and a 400MW-demonstration power plant will be built in the next five years. The IGCC system commonly has two parts: the topping cycle and the bottom cycle (HRSG and Steam Turbines). The topping cycle consists of the gasifier, the syngas cooling and clean-up subsystem, the air separation unit (ASU) and the gas turbine. The gas turbine is the key component of the topping cycle as well as the total IGCC system. The purpose of this paper is to study the influence on IGCC topping cycle performance of different degree of ASU integration incorporated with gas turbine and proportion of the re-injecting netrogen. In this paper, the key factors, which affect the topping cycle, are analyzed, a new idea−simultaneous optimization of the two opening variables−is presented and a mathematical model and the corresponding computing procedure are developed to realize the simultaneous optimization. This procedure is employed to calculate a typical commercial IGCC topping-cycle system, which is based on a slurry-feed gasifier, a low-temperature clean-up subsystem, a low-pressure ASU and a large gas turbine. The acquired results constitute useful information on the analysis and optimization of IGCC topping cycle performance during the system design phase.