This paper deals with the modeling and simulation of air-steam gasification of coal using Aspen Plus process simulator. The model is able to predict the gas products, tar concentration, and char yield where the prediction accuracy of the presented model has been validated against the experimental measurements and found to be in a good agreement. The effect of coal particle size and minimum fluidization velocity (U-min) in air-steam gasification is explored through key operating parameters such as reaction temperature, equivalence ratio, and steam/coal ratio. Maximum hydrogen yield of 50.5 g/kg coal was obtained at a temperature of 650 degrees C and steam/coal ratio of 0.7. Results were also showed that the particle size plays a key role in the process; smaller particles favor higher hydrogen yield.