Sustainable smart cities are increasingly turning to innovative technologies, such as Urban Digital Twins (UDTs) and Zero Energy Buildings (ZEBs), which offer transformative opportunities to enhance energy management and reduce environmental impacts. Despite significant progress, research on the integration of UDTs and ZEBs remains limited, and the lack of a unified framework hampers their potential to fully contribute to environmental goals in smart cities. Therefore, this study conducts a comprehensive systematic review of how UDTs and ZEBs can be integrated to strengthen climate change mitigation efforts in sustainable smart cities. It primarily aims to develop a novel framework that leverages their synergies for maximizing their combined potential to advance environmentally sustainable urban development. The study reveals key trends in the convergence of UDTs and ZEBs, emphasizing the growing role of Artificial Intelligence (AI), the Internet of Things (IoT), and CyberPhysical Systems (CPS), while also highlighting specific research patterns related to their synergistic interplay and its contribution to enabling the convergence. Moreover, it underscores the role of UDTs in enhancing the energy management and performance capabilities of ZEBs by improving energy efficiency, boosting renewable energy integration, and reducing carbon emissions through real-time monitoring, advanced data analytics, predictive maintenance, and operational optimization. Conversely, ZEBs provide real-time data and performance metrics that enhance UDTs' analytical and predictive capabilities. Furthermore, the study introduces a comprehensive framework that integrates UDT technical and operational indicators with ZEB performance indicators. However, technical, ethical, environmental, financial, regulatory, and practical challenges must be addressed and overcome for large-scale implementation. The novelty and contribution of this study lies in the development of a unique integrated framework that bridges the gap between UDTs and ZEBs, highlighting their collective impact on sustainable urban development, an area that has not been explored in previous review research. Theoretical and practical insights are discussed to inform researchers, practitioners, and policymakers, showing how these findings can shape future research directions, guide technological implementation, and influence policy decisions in advancing sustainable smart cities.