Nickel-catalyzed atmospheric-pressure hydrogenation and hydrogenolysis of trace light tar in biomass syngas were investigated with two different model biomass syngas compositions and benzene and toluene as model light tars. The optimum methane selectivity was obtained when the reaction temperature was between 623 and 673 K. The reaction rates were influenced by the partial pressures of hydrogen and carbon dioxide. Notably, carbon dioxide retarded the reactions of the aromatic hydrocarbons because of a competitive reaction. A Ni-CeO2 catalyst supported on Al2O3 (Ni-CeO2/Al2O3) enhanced the reactions, and almost all of the carbon atoms in benzene were converted into methane carbon when the hydrogen and carbon dioxide partial pressures were 40 and 30 kPa, respectively, at 673 K in the presence of the Ni-CeO2/Al2O3 catalyst. Furthermore, degradation of the catalysis was not observed for at least 80 min. The composition of this model gas is comparable to that of the syngas obtained by biomass gasification with pure steam. Therefore, trace light aromatic hydrocarbons can be converted into methane through the simple installation of a Ni-CeO2/Al2O3 catalyst bed at the optimum temperature zone downstream of the gasifier, without additional hydrogen, external heating, or secondary air when pure steam is used as a gasifying agent.