Safety analysis with system thermal-hydraulic codes plays and will continue to play a major role in the design, operation, licensing and safety of nuclear power plants. With stricter requirements from the nuclear authorities, the application of system codes for safety analysis tends to be a complex procedure that includes interactions with many neighboring disciplines such as core physics, chemistry, probabilistic safety analysis, etc. For many years in Paul Scherrer Institute in the framework of the STARS (Steady-state and Transient Analysis Research for the Swiss reactors) program, technical safety support for the nuclear authorities and industry has been performed within the Swiss Simulation Platform. The platform consists of a few integrated subsystems (or reference methodologies) such as Core Management System, Plant Management System (PMSYS), Fuel Management System, etc., supplemented with a common input/output interface and a common plant database. PMSYS is a code/model integrated plant system thermal-hydraulics methodology that is being developed specifically for plant system analysis. The paper reports the main principals of PMSYS development as well as the current system layout based on the SNAP Model Editor interface. The development process for a BWR-6 evaluation model is described together with some validation cases based on the real plant transients. The PMSYS interactions with the other subsystems of the Swiss Simulation Platform are also discussed.