Cosmic microwave background (CMB) radiation fills the universe almost uniformly and it has imprints of the primordial curvature perturbations. Observations of the CMB showed that to a large extent they are Gaussian [1]. However, there are hints of some non-Gaussianity in the CMB that is within the reach of experiments such as the Planck satellite. Different models of inflation predict different levels of non-Gaussianity. Comparison with the experiments could be used to falsify some models and single out the one describing the physics of inflation most correctly [2]. It has been found that preheating can produce substantial levels of non- Gaussianity in the massless model in the FRW backgroud [28]. In my thesis I present a detailed description of a refined calculation, which does not assume an FRW Universe but allows for the subhorizon inhomogeneous metric perturbations to interact with the scalar fields in a fully non linear fashion. Including the subhorizon metric perturbations in the dynamics is a physically much more accurate calculation. It allows for increasing the lattice size at will and therefore allows for including in the dynamics even those modes whose wavelengths are equal to or greater than the size of the horizon. I combined the lattice simulations with the separate universe approximation [5, 40]. The latter states that points in space separated by more than one Hubble distance cannot interact with each other and therefore evolve independently. The numerical integration of the Einstein Equations uses the BSSN scheme [7], a technique used to solve initial value problems in the 3+1 formulation of numerical relativity. I found that it is indeed possible to compute the superhorizon curvature perturbations at a constant density hypersurface including full metric perturbations. However, I also identified that accuracy improvements are needed to compare my results to the literature. I identified and successfully implemented a number of the improvements and presented some suggestions for the future research.