A simulation model was developed and validated to explore the energy flow distribution in plug-in hybrid electric vehicles (PHEVs) with integrated thermal management systems (ITMS), using a genetic algorithm to focus on components, subsystems, and the power system. The influence mechanism of temperature and state of charge (SOC) on the energy consumption of the ITMS was subsequently analyzed throughout the driving cycle. The main findings are: (1) Engines possess significantly greater potential for waste heat utilization compared to motors, with the thermal loss of the engine being 9.6 times greater than that of the motor. (2) Although the ITMS improves the engine energy utilization efficiency, considerable room for improvement remains. At -18 °C, the heat loss of the radiator and the heat used by the ITMS account for 14.6% and 7.7%, respectively. (3) A pronounced impact of low temperature and SOC on vehicle energy consumption was identified. Energy consumption increased by a factor of 2.14 as temperature decreased from 48 °C to -18 °C. The total energy consumption at SOC 35 was 1.44 times greater than at SOC 75. These findings offer valuable insights for optimizing energy flow in PHEVs equipped with ITMS.