Gasification is a multiphase process that converts the solid fuels into useful synthetic gas. Despite the potential of biomass gasification as a clean technique to produce a high caloric value syngas, it is not as well looked upon as its advantages suggest. In the present study, an ASPEN plus model of biomass gasification was developed with considering the kinetics of the reactions and tar cracking mechanism. Firstly, the model was validated against the experimental data in the literature. Next, a parametric study was done to investigate and evaluate the performance and produced gas composition of air–steam gasification of biomass. The effect of various operational parameters, such as the reaction temperature, equivalence ratio (ER) and steam/biomass ratio (S/B) on the gas product composition, products yield and the gasifier performance was investigated. The results showed that the increase of reaction temperature not only can enhance the H2 content in the gas stream, but inhibit the formation of tar. The achieved optimal conditions for production of maximum H2 content (16.18 vol %) were as follows: reaction temperature of 800 °C, S/B of 0.8 and ER of 0.211. [ABSTRACT FROM AUTHOR]