The partitioning of a diblock copolymer (AB) and a triblock copolymer (ABA and BAB) into a pore, at a condition similar to that employed in the experimental liquid chromatography at the critical condition (LCCC), was investigated by lattice Monte Carlo simulations with self-avoiding-walk chains. The B block is set at the predetermined critical condition where the partition coefficient of a homopolymer B has a least dependence on its chain length, and hence becomes chromatographically "invisible". The A block is set at the size-exclusion mode and is chromatographically "visible". The partition coefficients of these copolymers were compared with that of a homopolymer A with the same (total) length of the visible A block(s). The partition coefficient of a diblock, K-AB, was found to be larger than K-A, especially when the A block was short and the B block was long. The difference tends to vanish with an increase in the A block length or a decrease in the B block length. A smaller pore also tends to decrease the difference between the two. For the triblock copolymers, the partition coefficient for B-AB, K-BAB, was found to be larger than K-A and was almost equal to K-AB, but the partition coefficient of A(BA), K-ABA, was found be smaller than K-A. These simulation results are in remarkable agreement with experimental observations. As a result, the length of the visible block in diblock copolymer A(B) and triblock copolymer B-AB would be underestimated, and that in triblock copolymer A(BA) would be overestimated in LCCC. The origins of these observed differences in partition coefficients are explained.