Drillstring vibration is detrimental to drilling operations. It is crucial to understand the underlying mechanisms to circumvent these vibrations and to help improve drilling performance. This paper presents a six degree of freedom (DOF) finite element method (FEM) model to characterize the drillstring dynamics. In addition, a comprehensive bit-force model was developed and included as a boundary condition to the model, corresponding to the vibrations in axial, lateral, and torsional directions. This bit-force model considers the bottomhole assembly (BHA) eccentricity, mud damping, bit-rock interaction, and their coupling mechanisms. Simulation results have shown good agreement with field observations and experimental data in the literature. The utility of this modeling framework is demonstrated in the paper through case studies for normal operation, stick-slip vibration, and whirl vibration.