Proxy applications (proxy-apps) are basic tools for evaluating the performance of specific workloads on high-performance computing (HPC) systems.Since the development of high-fidelity proxy-apps, which exhibit similar performance characteristics as corresponding production applications, is labor-intensive, synthetic proxy-apps are created as a useful supplement to manually developed proxy-apps. To thoroughly resemble performance characteristics of HPC applications represented by Message Passing Interface (MPI) programs, we propose Siesta, a novel framework to automatically synthesize proxy-apps based on communication-computation traces. Given an MPI program, Siesta synthesizes parameterized code snippets to mimic computation behaviors in different execution periods, and combines the code snippets and MPI function records into an event trace. It then extracts program behavior patterns from the trace as grammars, and finally transforms the grammars into a synthetic proxy-app. We evaluate the proposed methods on representative MPI programs with various environments. The results show that our synthetic proxy-apps can precisely approximate the performance characteristics of MPI programs.