NOVELTY - The mass flow rate of a first reactant stream supplied to a fuel processor is controlled by determining mass flow factors of the first stream for the fuel processor based on at least one parameter related to a second reactant stream supplied to the fuel processor at a steady-state condition. A calculated mass flow rate is determined by taking into account the three mass flow factors.    USE - A method for controlling the mass flow rate of a first reactant stream supplied to a fuel processor, preferably syngas generator to produce a syngas stream used for heating or regenerating engine exhaust gas after-treatment devices or other hydrogen-consuming devices, e.g. fuel cells and/or combustion engine.    ADVANTAGE - The control regime is capable of responding rapidly, and with a reduced variance from the desired reactant supply value, during steady-state and transient conditions in load-following applications.    DETAILED DESCRIPTION - Controlling the mass flow rate of a first reactant stream, preferably fuel stream supplied to a fuel processor involves determining a first mass flow factor of the first stream for the fuel processor based on at least one parameter related to a second reactant stream supplied to the fuel processor at a steady-state condition. A second mass flow factor of the first stream is determined based on a rate of change to at least one parameter related to a second reactant stream supplied to the processor. A third mass flow factor of the first stream is determined based on at least one process parameter of the processor. A calculated mass flow rate is determined by taking into account the three mass flow factors. The steps are at least periodically repeated during operation of the fuel processor. An INDEPENDENT CLAIM is included for a system comprising a combustion engine (110) and a fuel processor for producing a hydrogen-containing gas stream.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic diagram of a combustion engine system.    Engine (110)    Conduit (116)    Syngas generator (119)    Controller (121)    Pressure sensor (123)