Process design and performance evaluation were made for medium-size gasification combined/cogeneration plant. Based on the designed plant process configuration, the effects of $NO_x$ reduction techniques on the $NO_x$ emission, the power output, the efficiency and the stability of plant are investigated by applying various $NO_x$ reduction methods such as unsaturated/saturated nitrogen injection and fuel saturation of gas turbine combustor. The $NO_x$ reduction by nitrogen injection is more remarkable than that by fuel saturation, and its effect can be more enhanced by using saturated nitrogen. In addition, the applications of $NO_x$ reduction techniques accompany the improvement of plant power output and efficiency with the decrease of $NO_x$ emission, while it can cause unstable gas turbine operation.