Laser Powder-bed Fusion (LPBF) is a common technique categorized as one of the Additive Manufacturing (AM) processes to efficiently fabricate complex geometries. The involvement of complex phenomena relating to laser and metal powder requires a thorough investigation to understand the complex multi-physics behind this process. Modeling and simulation tools shed light on predicting the temperature distribution and melt pool dimensions which have a significant impact on the quality of the final parts. In this study, a three-dimensional (3D) heat transfer model is developed to investigate the influence of the thickness of the printed thin-walls on melt pool dimensions and temperature distribution. The results indicate that the single-track simulation can predict the melt pool dimensions accurately and the calibrated model can be extended to the multi-track simulation for investigating the effect of thin-wall thicknesses on melt pool geometries. The simulation results demonstrate the evolution of melt pool geometries during the process. Due to the existence of heat accumulation during the process, decreasing the thicknesses of the thin-walls leads to enlarging the melt pool width significantly. Moreover, the simulation results show a higher temperature gradient during the LPBF process of thinner parts leading to a smaller grain size of the final microstructure. The validation of the simulation results showed the high capability of the model in predicting the transient temperature profile and melt pool geometries. The percentage difference between simulated and experimental melt pool width for thin-wall thicknesses 0.5 mm, 0.75 mm, and 1 mm are 7%, 7%, and 11%, respectively. Lastly, a process map has been provided to guide the selection of process parameters for printing thin-wall structures.