Transfer integrals (TI's) are essential parameters in the calculation of electron transport both in coherent and incoherent regimes. We show that TI's for polymer crystals can be obtained from first principles, starting from plane-wave density-functional calculations of the electronic structure in the local-density approximation, and propose methods at different levels of approximation. We demonstrate that special choices of single-chain states can be used very effectively as building blocks for the crystal electronic structure, thus allowing a deeper insight into the transport properties of molecular crystals. We apply this approach to polymer systems of great interest to molecular electronics, such as poly-para-phenylene-vinylene and polythiophene in different crystal packing morphologies, and show that it offers a very powerful tool to understand and design the impact of intermolecular interactions on conduction of organic crystals.