This study investigates and compares the experimental heat transfer performance and simulation via computational fluid dynamics (CFD) of graphene nanoplatelets (GNP) and water nanofluids GNP/water in the double-tube-type heat exchanger (DTHE). Tests were conducted with water/water and GNP/water fluids, with the nanofluid for the hot-fluid circuit and water for the cold-fluid circuit, with counterflow direction, varying the nanofluid concentrations by weight (wt%) at 0.0125%, 0.025%, and 0.050%, the operating temperature at 50 and 60 °C, and Reynolds numbers between 2000–6000. The results showed that 0.025 wt% GNP presented better thermal performance, with a 28% increase in the temperature gain. The 0.025 wt% GNP had slightly better performance for the Nusselt number (Nu), and the 0.05 wt% GNP had a slightly better thermal effectiveness. The comparison between the experimental values showed good agreement with those calculated by empirical correlations and the CFD model, with maximum and minimum relative error values of 9% and 1%, respectively, when the Petukhov equation was used.