The Spallation Neutron Source (SNS) project is a collaborative effort between Brookhaven, Argonne, Jefferson, Lawrence Berkeley, Los Alamos and Oak Ridge National Laboratories. Los Alamos is responsible for the design of the linac for this accelerator complex. The code PARMILA, developed at Los Alamos is widely used for proton linac design and beam dynamics studies. The most updated version includes superconducting structures among others. In recent years, some other codes have also been developed which primarily focuses on the studies of the beam dynamics. In this paper, we compare the simulation results and discuss physics aspects of the different linac design and beam dynamics simulation codes.