We present a method to calculate NMR chemical shielding tensors in condensed phases by means of a hybrid quantum mechanical/molecular mechanical (QM/MM) approach. We propose a modification of the conventional QM/MM technique, adding a general repulsive potential to the electronic interaction Hamiltonian. This universal potential is motivated by the absence of Pauli repulsion in standard interaction potentials that are based only on classical point charges. We apply the method to realistic systems composed of molecules with strong dipolar character, thus forming strong hydrogen bond networks. In particular, we present calculations for liquid water and a proton conducting organic crystal. The electrical field and direct contact effects of surrounding molecules play a crucial role in the NMR resonance lines of such materials. The results are in very good agreement with full quantum calculations as well as with experiment. Thus, this new combination of ab initio NMR chemical shift calculations with a QM/MM modeling of extended systems provides an improved tool for the analysis of complex biological and chemical systems, such as polymers and proteins.