This paper proposes a system-level circuit simulation framework for nanoplasmonic devices, and presents an example of the simulation of a plasmonic nanocircuit. The electronic design automation environment provides an equivalent circuit model library for several plasmonic metal-insulator-metal-based devices. The accuracy of the equivalent models for the plasmonic nanocircuit library is verified by using full-wave simulations and analytical equations. These models are then used to design an ultracompact Mach-Zehnder plasmonic modulator. It is shown that the voltage required to achieve a π phase shift Vπ in the modulator can be predicted by the simulator with reasonable accuracy. The optimized design of the modulator is also presented that reduces the value of Vπ according to the required specification.