In this paper, we present ion-optical simulation for the design studies of a new solenoid separator as part of the NEXT experiment. Our aim is to separate Neutron-rich, EXotic, heavy nuclei produced in multinucleon Transfer reactions (NEXT) from the primary beam and un-wanted by-products within the magnetic field. The goal of the simulation is to find the optimum arrangement of the components within the NEXT solenoid separator to achieve the highest transmission efficiencies of transfer products and a good suppression of background. In our simulations, we focused on two complementary reactions to produce transfermium isotopes and to produce neutron-rich nuclei around the neutron number N = 126. Transmission yields above 50% for the transfermium isotopes of interest were reached in the simulations. The simulated yields for N = 126 lie between 5% and 20%.