Fuel synthesis through coal and biomass gasification has the potential to provide a solution to the increasing demand for energy and transportation fuels. To theoretically understand the complex chemical processes in a gasifier and to identify the most influential parameters for syngas production, we developed a multiphysics model to simulate the gasification processes in a well-stirred reactor. This model is the first of its kind and considers detailed gas-phase chemistry, particle-phase reactions, radiative heat transfer, as well as full coupling between the two phases at various scales for mass, species, and energy exchange. The gas-phase reactions use the detailed chemistry GRI-Mech 1.2, including 177 elementary reactions and 31 species, as well as variable thermodynamic and transport properties. Four surface reactions were considered and the reaction rates were simulated by the diffusion-kinetics model with consideration of boundary layer diffusion. A random pore model was used to account for the evolution of the char porous structure and its impact on gasification rates. A numerical code was developed to solve the gas-phase and the particle-phase governing equations. Numerical simulations were conducted to understand the gasification process and the effects of particle size, porous structure, radiative heat transfer, pressure, O-2 concentration, and H-2 addition on gasification performance. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.