In this paper, a thermochemical equilibrium model is used to predict the performance of a downdraft biomass gasifier. Numerical results are shown to be in good agreement with those of the experiments. Different biomass materials are tested using the model, and forest residual is shown to be the most energetic one. For this material, the gasification temperature, syngas composition and calorific value are calculated. The effects of moisture content, air/fuel ratio, air inlet temperature and steam/fuel ratio are also investigated. The air inlet temperature is found to be the only way to increase syngas calorific value and cold gas efficiency. The steam/fuel ratio, on the other hand, plays a key role in controlling the gasification temperature and H2/CO ratio.