Grain-boundary melting in a lattice-gas model of a bicrystal is studied by Monte Carlo simulation using the grand canonical ensemble. Well below the bulk melting temperature Tm, a disordered liquidlike layer gradually emerges at the grain boundary. Complete interfacial wetting can be observed when the temperature approaches Tm from below. Monte Carlo data over an extended temperature range indicate a logarithmic divergence w(T)∼-ln(Tm-T) of the width of the disordered layer w, in agreement with mean-field theory.