Industrial gas turbines (GTs) have advanced and numerous changes in both use, materials employed and design boundaries have occurred. The constraints aimed at lowering NOx emissions for improving the footprint, have driven designers to increase the firing temperatures and to look for improved cooling systems of GTs nozzles. These factors led to more severe operating conditions for hot gas path components and to the need for more accurate and comprehensive thermo-mechanical models for life assessment purposes. Within this context, a Lemaitre-Chaboche viscoplastic model has been coupled with a modified θ-projection creep model for MAR-M-247 material to account for steady-state creep effects in a finite element transient analysis (FEA). These material models have been applied on a cooled, first stage nozzle for a recently developed high-efficiency gas turbine. A verification assessment has been performed on a overfired, off-design transient analysis. The viscoplastic model developed showed the capability of predicting an accurate initiation zone of cracking. The FEA employing the captured inelastic behaviour predicted a high stress state in the location where an experimental crack nucleated. Furthermore, the model demonstrates the capability of using the Lemaitre-Chaboche plastic model coupled with the θ-projection creep model to predict reasonable inelastic strains for the entire lifetime of a gas turbine.