Open rack vaporizer (ORV) is a type of commercial heat exchanger widely used in large gasification plants for base load Liquefied Natural Gas (LNG) receiving terminals. The heat source is seawater which is abundant and easily obtained. The aim of this study is to build a relatively accurate model of quantitative calculation for heat transfer in ORV. The heat transfer correlations and calculation method are described and discussed for inside and outside of the tube in ORV, then the calculation results are compared with the data of a practical case. Meanwhile, numerical simulation is established by FLUENT to verify the heat transfer of the LNG and seawater. The deviation between the correlation calculation model and the numerical simulation is within 10%. In addition, the effects of different operating parameters on heat transfer are investigated separately. Both the inlet pressure and the mass flow of methane have significant effects on the thermal performance of the tube, and the effects of the inlet temperature of water is rather slighter. The determination for the optimal correlation and calculation model may provide guidance for the design of ORV.