We present the first theoretical estimates for thermoelastic properties of the noncrystalline domain (the "interlamellar phase") of semicrystalline polyethylene obtained by Monte Carlo simulations. The interlamellar phase is prescribed to be thermodynamically metastable, with the constraints that it have an average density less than that of the crystal and that it be bounded by two static crystalline lamellae oriented with the {201} crystal plane parallel to the interface. Polyethylene was modeled using a realistic united atom force field with inclusion of torsional contributions, and the results are compared to those of prior studies that used a freely rotating chain model. Parallel tempering between 350 and 450 K was used to simulate several isochoric/isothermal ensembles simultaneously and efficiently, from which the heat capacity, thermal expansion coefficients, Gruneisen coefficients, and the elastic stiffness tensor were determined at atmospheric pressure. The noncrystalline interlamellar phase exhibits properties intermediate between that of the semicrystalline solid and the amorphous melt.