High-pressure and high-temperature in-gasifier removal of H2S using CaCO3 sorbent powder is studied in a differential-bed flow-through reactor at pressures of up to 2 MPa and temperatures of up to 900 degrees C. The CaCO3 sulfidation kinetic data are obtained under conditions closely simulating the pressurized fluidized bed gasifier conditions of an integrated gasification combined cycle system. The effects of total pressure, H2S partial pressure (2-5 kPa), reaction temperature, CO2 and H-2 partial pressures, particle size (16-200 mu m), and CaCO3 surface area on the extent of H2S removal and solid conversion are determined. The gasifier pressure is found to have a negative effect on the sulfidation reaction. The sulfidation reaction is first order with respect to H2S partial pressure. The conversion data of the sulfidation reaction are analyzed using the grain model, with the overall reaction rate being controlled by chemical reaction at the interface and by diffusion through the product layer. At higher conversions, the reaction is found to be product layer diffusion controlled with an apparent activation energy of about 48.7 kcal/mol. The high-pressure sulfidation kinetic data obtained in this study would be useful in understanding and optimizing the in-gasifier H2S capture using CaCO3 sorbents.