The nuclear magnetic shielding polarizabilities of N-2, F-2, CO, HF, HCl, HCN, HNC, and C2H2 have been computed at the coupled-cluster singles and doubles (CCSD) level augmented by a perturbative treatment of triple excitations [CCSD(T)] using gauge-including atomic orbitals and a mixed analytical-numerical approach based on the numerical differentiation of analytically evaluated shieldings. Extended basis sets have been employed to ensure results close to the basis set limit, and convergence of the electron-correlation treatment is investigated.