Thermo-chemical conversion of biomass has been regarded as one of attractive routes of producing the clean and environmental friendly bio-fuels. Downdraft biomass gasification process has been served as a key technology to provide the bio-syngas with high quality, which can be used to produce the renewable liquid transportation fuels through Fischer Tropsch Synthesis in Mississippi State University. In order to provide the performance data of the integrated Biomass to Liquid system and future process optimization, a comprehensive model of the downdraft biomass gasification process based on Aspen Plus by minimizing Gibbs free energy with restricting chemical reaction equilibrium in the gasification reduction zone has been developed. The model was successfully validated with the experimental data from the hardwood chips gasification. Sensitivity analysis was also performed to investigate the effects of gasification temperature, equivalence ratio, and biomass moisture content on the quality of bio-syngas. All the investigated factors have been found with a significant effect.