Data dissemination in vehicular ad hoc networks (VANETs) has attracted researchers' attention in recent years. However, due to high vehicle density in urban areas and increasing data volume, wireless interference seriously impairs data dissemination performance of a VANET. To alleviate the interference problem, we propose a cross-layer design to exploit both spatial and temporal reusability of wireless spectrum in VANETs. In particular, the proposed cross-layer design integrates power control in the physical layer, time division in the MAC layer and data routing in the network layer. To implement the cross-layer design, we formulate an interference-aware power-control (IAPC) problem with the objective of maximizing data throughput in a VANET. For solving the IAPC problem, an integer linear programming formulation is proposed to derive the optimal solution. In addition, an efficient heuristic algorithm, named IAPC-Solver, is proposed to achieve a near-optimal solution. Extensive simulation experiments have been conducted on a widely adopted testbed, and the experimental results illustrate that the cross-layer design together with the proposed algorithms are effective for improving data dissemination performance of a VANET.