Integrated gasification combined cycle (IGCC) plants with CO2 capture have strong potential in the future carbon-constrained world. In these plants, the CO2 and COS content of the syngas at the inlet of the acid gas removal process should be within certain limits in order to satisfy the environmental limits on sulfur and CO2 emissions. To satisfy these constraints, the syngas from the gasification process is passed through water gas shift reactor(s). A premium is placed on sulfur-tolerant catalysts since the syngas may contain COS and H2S. In comparison to the sweet-shift process, the sour-shift process results in higher overall efficiency because of the higher temperature of the feed syngas and requirement of less additional steam for the shift reactions. The optimal operating conditions and the dimensions of the sour shift reactors can be obtained by considering the effect of a number of key variables. With this motivation, a 1-D mathematical model of a sour water gas shift (WGS) reactor has been developed by considering mass, momentum, and energy conservation equations. The experimental data available in the open literature are reconciled for measurement errors after gross errors are removed and then used to obtain the rate parameters. Subsequently, the developed model is used to study the performance of the WGS reactor as part of an IGCC plant with CO2 capture. The results presented in this paper are very useful in designing, analyzing, and operating the sour water gas shift reactors.