A reformate clean-up reactor. The reactor takes a reformate stream and passes it through multiple subreactors that are integrated into a common reactor housing to reduce reformate stream by-product concentration prior to use of the reformate in a fuel cell. The reactor includes a gas shift subreactor to promote the conversion of carbon monoxide to carbon dioxide, a gaseous diffusion membrane subreactor to provide a hydrogen-rich portion of the reformate stream, and a methanation subreactor to convert carbon monoxide into methane and water. In applications where space for a fuel cell system is limited, the integration of the clean--up devices into a common housing provides significant improvements in structural and volumetric efficiency. Moreover, in at least one embodiment of the present invention, the juxtaposition of the gaseous diffusion membrane and the gas shift reactor improves membrane robustness. The purpose of the abstract is to enable the United States Patent and Trademark Office and the public generally to determine from a cursory inspection the nature and gist of the technical disclosure, and is not to be used for interpreting the scope of the claims.