The formation of CO as an intermediate in the conversion of CO2-H2 to CH4 and its strong binding on dispersed Ru, Co, and Ni nanoparticles inhibit rates of CO and CH4 formation but to different extents. CO2 conversion rates decrease and CH4 selectivities increase as CO concentration gradients evolve axially along the catalyst bed and radially within diffusion-limited porous aggregates. These trends and their interpretation in terms of the identity and kinetic relevance of surface-catalyzed elementary steps enable mechanism-based strategies for selectivity control through the purposeful introduction of CO pressures into inlet CO2-H2 streams. This strategy exploits the stronger CO inhibition of its formation (from CO2) than its conversion (to CH4), which causes the selective inhibition of CO2 conversion relative to CH4 formation. The presence of CO at levels accurately prescribed by the mechanism-derived rate equations, similar in functional form on Ru, Co, and Ni nanoparticles, and by diffusion-convection-reaction models that account for CO gradients at the bed and aggregate scales led to the exclusive formation of CH4 and to the elimination of CO gradients at both scales, as evident from measured rates and selectivities for CO2-H2 reactions on Ru, Co, and Ni nanoparticles over a broad and practical range of temperature (483-573 K), reactant pressures (4-1100 kPa CO2; 8-820 kPa H2), and nanoparticle diameter (2-30 nm). This mechanism-based strategy enables the exclusive formation of CH4 from CO2-H2 reactants, irrespective of reaction conditions or nanoparticle composition (Ru, Co, Ni) and size, without requiring complex catalyst architectures or intricate synthesis protocols.