To reflect the complexity of the energy transition, a current challenge in modeling national energy transition pathways is to combine high resolution in time, space, techno-economic and sector coupling details in a single model. To address this challenge, the paper discusses the simulation-based optimization approach used in the energy system model REMod as an alternative to linear optimization. The REMod model uses a simulation of the energy system coupled with a black-box optimization algorithm to optimize the transformation path. To limit the computational complexity, taking into account the hourly operation of the energy system along the whole transformation path, various aspects have to be considered: performance of the simulation, choice of the optimization algorithm and selection of the termination criterion and population size. The model employs a novel method of endogenous interpolation to incorporate the entire transformation path into the objective function, and it can be evaluated in parallel. This allows to increase the complexity both in technological details and in geographical resolution, leading to a model with high resolution both in sector coupling and technological details, as well as in time and space.