Nonthermal plasma created in a catalyst-packed bed reactor was applied to the conversion of carbon oxides (CO and CO2) into methane. The methanation was performed over a Ni/alumina catalyst with the molar ratios of H2/CO and H2/CO2 at 3.0 and 4.0, respectively. The present work focused on the effect of nickel loading (0–12.5 wt.%) on the methanation rate over the temperature range of 200–300 °C. The applied voltage for creating nonthermal plasma was 6.4–10.3 kV (operating frequency: 1 kHz). For both CO and CO2, the behavior of the methanation in the plasma-catalytic reactor greatly depended on the applied voltage, resulting in higher conversion efficiencies at higher voltages. There was optimal nickel content of 2.5–5.0 wt.% where the plasma effect on the methanation rate was maximized. It is believed that the plasma created in the catalytic reactor can change the rate-determining step by accelerating the dissociation of carbon oxides adsorbed on the catalyst surface.