An advanced transport reactor gasifier was operated using three different coals: Illinois #6 bituminous, Wyodak Powder River Basin subbituminous, and a Sufco Utah bituminous coals. A steady-state model was developed to evaluate the relative contributions of coal combustion and gasification of coal/char in this high-throughput reactor. The tests employed in-bed calcium-based sorbent for sulfur capture. The model was based on elemental mass and energy balances and assumed instantaneous devolatilization and combustion and kinetically limited gasification reactions in a continuously stirred tank reactor. The experimental results on the reaction of these three coals were compared with model predictions. The simulated results compared favorably with the experimental results (e.g., gas composition, carbon conversion, and reactor temperature). The bituminous coal data were accurately simulated as substoichiometric combustion processes. The more reactive subbituminous coal exhibited significant gasification conversion over the operating conditions tested. The subbituminous coal was found to react 20 times faster than reported from laboratory kinetic studies. The model was used to predict the performance of the gasifier, including the carbon conversion, sulfur capture, and composition and flow rate of product gas, based on operating conditions and input streams. Sensitivity studies on the coal feed rate, steam/coal ratio,heat loss, pressure, gas velocity, and coal reactivity were conducted. These simulations were used to compare the response of coals gasified to those combusted substoichiometrically, to evaluate the optimum operating conditions and to predict the performance in larger-scale units with less heat loss.