The devolatilization process and the moisture content in coal and biomass play an important role on the gasification performance of these fuels. To theoretically understand the complex chemical processes in a gasifier, we developed a multiphysics model to simulate the gasification processes in a well-stirred reactor. This model is a first-of-its-kind: it considers detailed gas-phase chemistry, drying and devolatilization kinetics, particle-phase reactions, boundary layer diffusion, pore evolution, as well as full coupling between the two phases at various scales for mass, species, and energy exchange. Numerical simulations were conducted using an in-house code to understand the comprehensive gasification process, and the focus was on the effects of devolatilization and moisture content. Sensitivity analysis was performed to identify the most influential parameters among various chemical and physical processes on the overall gasification performance (conversion time and syngas production). The results show that the syngas yield is most sensitive to the reaction rates of carbon-steam and carbon-CO2 reactions; the rates of drying and devolatilization have little impact on the syngas composition. The coal conversion time is most sensitive to the heat transfer rates including both radiation and convection, and its secondary sensitivity is to the reaction rate of the carbon-steam reaction. Lastly, the coal conversion time increases with increasing moisture content. This is because high moisture content causes a decrease of temperature, which reduces the reaction rates.