This paper describes a numerical and experimental investigation in support of the ARIES-CS divertor design, which selected a modular, helium-cooled, T-tube design that can accommodate a peak heat load of 10 MW/m2. Numerical analyses were carried out using the FLUENT computational fluid dynamics software package to evaluate the thermal performance of the divertor at the nominal design and operating conditions. Sensitivity studies were also performed to determine the effect of variations in geometry and operating conditions resulting from manufacturing tolerances and/or flow maldistribution between modules. The results indicate that the selected design is "robust" with respect to such anticipated variations in design and operational parameters and that a peak heat flux of 10 MW/m2 can be accommodated within the constraints dictated by material properties. Extremely high heat transfer coefficients [>40 kW/(m2K)] were predicted by the numerical model; these values were judged to be "outside the experience base" for gas-cooled engineering systems. Hence, an experimental investigation was undertaken to verify the results of the numerical model. Variations of the local heat transfer coefficient within an air-cooled, geometrically similar test module were measured at the same Reynolds number as the actual helium-cooled divertor. Close agreement between the model predictions and experimental data was obtained. The results of this investigation provide added confidence in the results of the numerical model used to design the ARIES-CS divertor and its applicability to other gas-cooled high-heat flux components.