Field-theoretic simulation (FTS) offers an efficient means of predicting the equilibrium behavior of high-molecular-weight structured polymers, provided one is able to deal with the strong ultraviolet (UV) divergence that occurs at realistic molecular weights. Here melts of lamellar-forming diblock copolymer are studied using a Monte Carlo version (MC-FTS), where the composition field fluctuates while the pressure field follows the mean-field approximation. We are able to control the UV divergence by introducing a new effective Flory-Huggins interaction parameter, chi(e), thereby permitting MC-FTS for molecular weights extending down to values characteristic of experiment. Results for the disordered-state structure function, the layer spacing and compressibility of the ordered lamellar phase, and the position of the order disorder transition (ODT) show excellent agreement with recent particle-based simulation. Given the immense versatility of FTS, this opens up the opportunity for quantitative studies on a wide range of more complicated block copolymer systems.