A simplified process model was developed to simulate a catalytic membrane water gas shift (WGS) reactor. A number of computer simulations were conducted to determine the potential of increased carbon monoxide (CO) conversion in WGS reaction due to simultaneous separation of product hydrogen (H-2) from the reactant mixture. Gas separation factors based on Knudsen diffusion were used in these simulations to assess the feasibility of inorganic and ceramic membranes in a high-temperature, high-pressure (HTHP) coal gasification environment. The simulations indicated that although the increase in CO conversion and hydrogen concentration in a single membrane reactor stage was significant, a multistage membrane separation system would be needed to increase the hydrogen product concentration above 90%. As expected, increasing the feed pressure to permeate pressure ratio was found to increase the CO conversion and the product hydrogen concentration. At low feed to permeate pressure ratios, the model predicted a much better membrane reactor performance with a countercurrent feed and permeate flow scheme when compared with a concurrent flow scheme. The membrane performance, of course, depends strongly on the gas separation factors. With gas separation factors lower than the ideal Knudsen diffusion separation factors (e.g. H-2 to CO2 separation factor of 2 instead of the ideal Knudsen value of 4.7), the model simulation predicted a much smaller increase in CO conversion and product hydrogen concentration.