Multilayered power-ground (PG) planes are commonly used to supply power to circuits in high-speed electronic packages and printed circuit boards (PCBs). In this paper, we propose an efficient integral equation equivalent network method to simulate the interference produced by the rerouted return currents of signal traces inside such PG planes. The integral equation is created based on the geometrical features of the PG planes. Therefore, it could reduce the computing time and will be maintaining good accuracy. By discretizing the integral equation, a multiport equivalent network model is derived for each layer of the PG planes. Their [$Z$] matrices are then obtained. These [$Z$] matrices can be smoothly integrated with signal traces and other circuit models of lumped components mounted in the package and on the PCB, so that the system-level signal integrity and power integrity can be simulated efficiently.