Systems and methods for producing hydrogen gas with a fuel processing system that includes a hydrogen-producing region that produces hydrogen gas from a feed stream and a heating assembly that consumes a fuel stream to produce a heated exhaust stream for heating the hydrogen-producing region. In some embodiments, the heating assembly heats the hydrogen-producing region to at least a minimum hydrogen-producing temperature. In some embodiments, the feed stream and the fuel stream both contain a carbon-containing feedstock and at least 25 wt % water. In some embodiments, at least one of the feed and fuel streams contain at least one additional component. In some embodiments, the feed and fuel streams have the same composition. In some embodiments, the feed and fuel streams are drawn or obtained from a common source or supply, and in some embodiments as a liquid stream that is selectively apportioned to form the feed and fuel streams.