We apply the replica methodology and the replica Ornstein-Zernike equations to calculate adsorption isotherms on an energetically heterogeneous surface. The surface is modeled by quenching a two-dimensional "fluid of centers." After the quenching, we study gas adsorption using the replica Ornstein-Zernike equation with the hypernetted chain closure. The results of theoretical predictions are compared with the data obtained from the Grand Canonical Monte Carlo simulation, and the obtained agreement is satisfactory.