Selective CO methanation (CO-SMET) in the reformate gas, containing (vol.%): 1.0 CO, 65H(2), 10H(2)O, 20 CO2 with He as balance, was investigated over a number of nickel-ceria catalysts: treated with NH4Cl before (Ni/CeO2(Cl*)) and after (Ni(Cl*)/CeO2) Ni deposition, prepared by Cl-containing Ni precursor (Ni(Cl)/CeO2) and Cl-free one (Ni/CeO2). The effect of residual chlorine, originating from the catalyst preparation procedures, on the activity and CO selectivity of the samples was demonstrated. It was shown that all Cl-containing Ni/CeO2 catalysts provided efficient CO cleanup. They provided the removal of CO from reformate gas to the level below 10 ppm with a selectivity up to 90%. The catalyst characterization by BET, XRD, XPS, HAADF-STEM, EDX-mapping, FUR in situ and CO chemisorption techniques revealed that the decrease in chlorine content in the order Ni(Cl)/CeO2 >= Ni(Cl*)/CeO2 > Ni/CeO2(Cl*) was accompanied by the increase of Ni dispersion that most likely provided high performance of Ni/CeO2(Cl*) in CO-SMET. The turnover frequencies of Ni surface atoms as well as activation energies in CO methanation were practically similar for all studied catalysts, indicating that Cl did not influence catalyst's activity and CO methanation proceeded by similar ways over Ni surface in both Cl-free and Cl-containing samples. The advanced performance of Cl-containing catalysts was associated with the inhibition of undesirable side reaction of CO2 methanation. The chlorine doping effect was attributed to the blockage of surface Ce3+-coupled oxygen vacancy sites by CeOCl species that inhibited ceria-assisted CO2 activation and hydrogenation. The CeO2 treatment with NH4Cl before Ni deposition allows to prepare highly active and selective CO-SMET catalyst with high nickel dispersion and Cl-modified ceria surface.