The aim of this study was to predict the effect of operating conditions (temperature and equivalence ratio (ER)) on product distribution and gasification performances during air-steam gasification of MSW. To predict the gas composition, an Aspen Plus model was developed based on Gibbs free energy minimization. The root-mean-square error (RMSE) and model deviation percentage (MDP) was used to evaluate the accuracy of the model. Validation showed good agreement of the reported and predicted values for the gas caloric value. Increasing the temperature from 650 to 850(degrees)C led to an increase in H-2 content (3.97-4.27 vol %) in the gases. An optimal temperature of 650 degrees C resulted in the highest syngas lower caloric value (LHV) and lowest CO2 content.