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A hybrid electromagnetic (EM) circuit simulator is proposed for incorporating multiport lumped circuit networks through their admittance matrices into the discontinuous Galerkin time-domain (DGTD) method. The admittance matrix in the Laplace domain can be derived analytically or obtained from network parameters such as S-parameters in the frequency domain. To convert frequency-dependent S-parameters into the admittance matrix in the Laplace domain, the vector-fitting technique is employed to facilitate the mapping process. The computational domain of interest is split into two subdomains. One is the EM part solved by the DGTD, and another is the circuit part modeled by the basic I-V relationships in the time domain. The couplings between the EM and circuit parts happen at lumped ports where the port voltages and currents are solved via these coupled systems. Due to the local properties of DGTD operations, only small coupling matrix equation systems are involved. To further improve the efficiency, local time-stepping strategy is included. To show the validity of the proposed simulator, several numerical examples are presented and compared with results from other references.