Nickel based Alloy 617 is one of the leading candidate materials for intermediate heat exchanger (IHX) of the next generation nuclear plant (NGNP). The IHX is anticipated to operate at temperatures between 800-950ºC, which is in the creep regime. In addition, system start-ups and shut-downs will induce low cycle fatigue (LCF) damages in the IHX components. Hence, designing IHX using Alloy 617 for NGNP construction will require a detailed understanding of the creep-fatigue and ratcheting responses. In this study, a broad set of multiaxial creep-fatigue and ratcheting experiments are performed and the results are critically evaluated. Experiments are conducted by prescribing multiaxial loading histories in axial and shear, stress and strain space at 850ºC and 950ºC with different strain rates and strain amplitudes. Experimental results revealed that the axial strain ratcheting and cyclic hardening/softening responses of Alloy 617 vary significantly with temperature levels, strain rates and strain amplitudes indicating the dependence of creep-fatigue and ratcheting responses on these parameters. A unified constitutive model (UCM) based on the Chaboche framework is developed and validated against the multiaxial experimental responses. UCM simulated responses are compared against the experimental responses for determining the current state of material modeling and if modeling improvement are needed for IHX design applications.