Tungsten (W) is a candidate structural material for next-generation nuclear reactors due to its high thermal conductivity, reduced swelling under irradiation, low thermal expansion, and high sputtering resistance. Single-crystal (SC) models (i.e., [540], [320], [310], [520], [910], [010], [810]) and models with grain-boundary (GB) (i.e., Σ13(320), Σ5(310), Σ29(520), Σ41(540), Σ41(910), Σ65(810)) were built, simulated, and tested to calculate the lattice thermal conductivity (κ) over a temperature range of 300–1500 K. The influence of temperature, grain-boundary energy, and misorientation angle on κ were investigated. The SC-models showed higher κ than the GB-models for all tested temperatures (e.g., 10.93–15.03 W·m−1·K−1 for the SC-models vs. 5.62–12.02 W·m−1·K−1 for grain-boundary models at 300 K). The model with [010] orientation exhibited the highest κ among all the SC-models. In grain-boundary models, Σ41(910) with the lowest misorientation angle and grain-boundary energy had the highest κ (i.e., 12.02 W·m−1·K−1), while Σ5(310) with a relatively high misorientation angle and grain boundary energy had the lowest κ (i.e., 5.62 W·m−1·K−1). In models with grain-boundary, increasing the misorientation angle steadily decrease κ until it reaches ~36°, where κ achieved a plateau around ~5.9 W·m−1·K−1. The grain-boundary became more disordered beyond ~36°, where the phonon scattering is maximum and κ became nearly constant. In models with low angle grain boundaries, the decrease in κ with increasing temperature was significant; while, in models with high angle grain boundaries, the decrease in κ with increasing temperature was minimal.