A robust, single-particle gasification model is presented that is capable of predicting char particle behavior in environments established in typical fluidized-bed and entrained-flow gasifiers. It employs a heterogeneous reaction mechanism that describes char reactivity to CO2, H2O, and O-2 in the presence of H-2 and CO, gases that inhibit char reactivity. An effectiveness factor-Thiele modulus (eta-phi) approach is used to determine overall conversion rates when species concentration gradients exist inside particles, which occur at high particle temperatures when chemical reaction rates and mass transport rates through particle pores become competitive. In the approach taken, a eta-phi relation is determined for each reactive gas (CO2, H2O, and O-2) and deviations from first-order behavior are correlated with the concentrations of the inhibitors (CO and H-2). A mean effectiveness factor is defined on the basis of the individual species effectiveness factors and used in a mode of conversion model that governs the variations in particle size and apparent density during char conversion. In this paper, the pertinent model equations are presented, with focus on the effectiveness factor-Thiele moduli relations. The model is shown to be useful in identifying rate-limiting processes during char conversion in gaseous environments varying in temperature and composition. It serves as a tool that can be used to help design efficient coal- and biomass-fired entrained-flow and fluidized-bed gasifiers as well as combustors.