Landfill gas is a source of CH4 emission, also rich in CO2 (up to 50 vol %). It can be upgraded to renewable natural gas (RNG) by separating CO2 and impurities. Alternatively, the CO2 contained in biogas can be converted into CH4 via the Sabatier reaction, using H2 generated by water electrolysis. For industrial applications, it is beneficial to eliminate the energy-intensive CO2 separation step, converting biogas to RNG directly. In this work, a series of 0.02–1 wt % Ru/γ-Al2O3 catalysts were prepared by wet impregnation and evaluated for a single-pass conversion of CO2–CH4 mixtures. Through the catalytic performance evaluation and characterization studies, the optimal Ru loading was identified as 0.1–0.5 wt %. For these catalysts, CO2 conversion of 80–87% was achieved at 450 °C and 90 000 mL/g·h, maintaining 95–99% selectivity to CH4 production. These catalysts also showed excellent stability over 100 h on stream, while maintaining 99–100% CH4 selectivity.