It may become attractive in the future to operate fuel-related chemical reactions, with H2 as reactant, under flexible load conditions when H2 is produced through H2O electrolysis using fluctuating renewable energy. In this way, the size of the H2 storage device connected to the electrolysis cell can be reduced and, consequently, its investment costs. The design of a flexible reactor depends on the characteristics of the catalyst and the chemical reaction. CO2 hydrogenation over Fe catalysts to short-chain hydrocarbons, which can be used to adjust the heating value of substitute natural gas, is investigated as example reaction. A specific reactor design for flexible operation with gas recycling is a consequence of the constraints resulting from product partial pressures and reaction temperature. The experimental and mathematical methods developed can be applied to other fuel-synthesis processes.