1. A method of controlling the operation of a hydrogen-producing fuel processing system that includes a reactant delivery system, the method comprising: providing a carbon-containing feedstock stream from a carbon-containing feedstock source; supplying a steam stream from a steam source; combining the carbon-containing feedstock stream and the steam stream to produce a combined feedstock stream; measuring a temperature of the steam stream, a temperature of the carbon-containing feedstock stream, and a temperature of the combined feedstock stream; controlling a steam-to-carbon ratio within the combined feedstock stream, wherein the controlling includes controlling based at least in part on the temperature of the steam stream, the temperature of the carbon-containing feedstock stream, and the temperature of the combined feedstock stream; and applying the combined feedstock stream to a hydrogen-producing region of the hydrogen-producing fuel processing system to produce a mixed gas stream that includes hydrogen gas as a majority component.