This paper presents a dynamic model of a three-pressure-stage Heat Recovery Steam Generator (HRSG) system. It is developed on the Siemens T3000 plant monitoring environment. The multi-physics mathematical model captures essential physical phenomena of the HRSG system such as thermodynamics, heat transfer, phase change, fluid dynamics, etc. as well as their couplings. Fast simulation of the dynamic model is achieved and real-time execution is feasible. Aside from heat exchange elements such as economizers and superheaters, the critical components of the model are the high, intermediate, and low pressure boilers. Here, phase transitions and widely different operating pressures pose unique computational challenges and demand accurate modeling of mass and energy balance during unsteady conditions. A two-mode dynamics has been implemented in modeling the boilers. The model switches between these modes during transients. Through collaboration with Siemens Energy Inc., the model’s transient behavior and steady values have been validated with selected transient startup-data from a real HRSG.