An in-depth analysis of oil-cooled and naturally ambient air-cooled fixed bed reactors for catalytic methanation of a feedgas containing CO and CO2 has been performed. Combined investigation of modelling and experiments showed, that small tube-to-pellet diameters ratios and optimized reactor cooling are beneficial for high-capacity CO/CO2 methanation. Very good model accuracy was proven with a 1D approach for small diameter reactor pipes. It is shown that the reactor design sweet spot under consideration of input gas capacity, methane output concentration, catalyst degradation and pressure loss can be assessed by the experimentally validated reactor model. The study reveals insights to the mechanism of combined CO and CO2 methanation showing that initial CO methanation is kinetically limited, while subsequent CO2 methanation is ruled by the kinetics of the reverse water gas shift reaction. Finally, this works aim is to provide a design strategy for effective and cheap high-capacity CO/CO2 methanation reactors for industrial scale using commercial pellet catalysts in oil-cooled tube-bundle-reactors.