This paper presents an integrated modeling approach that combines the legacy urban runoff and the legacy storm sewer models with data exchange at runtime to simulate sewer flows with real-time rainfalls. A software system called IRES has been developed to demonstrate the proposed approach. IRES uses the RUNOFF and the EXTRAN block of Storm Water Management Model (SWMM) as the legacy urban runoff and storm sewer models, respectively. This study uses the Open Modeling Interface and Environment (OpenMI) framework, which is the standard interface for model integration, as the platform for dynamic data exchange in IRES. The Jhong-Kang drainage system located in Taipei, Taiwan, is taken as the case study to verify the proposed approach. Simulated results show that IRES is valid, and the proposed approach can be successfully applied to legacy model integration with dynamic data exchange.