Molecular simulations are powerful and predictive tools for studying polymers in bulk and at interfaces. However, simulation of high molecular weight entangled polymers is not straightforward, because of the broad ranges of time and length scales involved in their motions. Here we present a hierarchical simulation methodology for the calculation of the dynamical and linear viscoelastic properties of polymer melts. At the finest level, chains are described via an atomistic model. At a moderately coarse-grained (mCG) level, few atoms (e.g., one monomer) of the chain are lumped into one CG bead. The mCG potentials are derived based on the finer level atomistic model. Thorough the mCG model, moderately entangled chains can be simulated. Also, the results of the mCG model are used to parameterize a highly coarse-grained slip-spring model in which several monomers of the chain are mapped into one bead. We also present the results of atomistic simulations for the behavior of polymer melts in the vicinity of nanoparticles (NPs). Interfacial packing of polymer chains close to the NP surface, statistics of the adsorbed chains, and interface induced dynamical heterogeneities are discussed.