This paper proposes an optimal energy flow technique for the integrated electricity and gas systems (IEGS) with injections of alternative gases. Firstly, we develop a novel optimal energy flow model of IEGS with alternative gases, where the physical characteristics (e.g., specific gravity) of the gas mixtures are modeled as variables to reflect the impacts of alternative gas injections more accurately. Security indices are introduced to restrain the gas composition variation. Then, convex optimization techniques are tailored to transfer the original highly nonlinear and nonconvex optimization problem into a tractable form. An advanced sequential programming procedure is proposed with self-Adaptive convergence criteria to better balance the feasibility and convergency. Finally, the IEEE 24-bus RTS and Belgium gas transmission system, and the practical 160-bus Britain gas system are used to validate the proposed techniques. It can be observed from numerical studies that the computation efficiency of the proposed solution methods is 96.44% faster than traditional mixed-integer nonlinear solvers. The injection of alternative gas can cause up to 4.32% variations in the nodal gas pressure. Nonetheless, under various uncertainties from renewable generation, load level, etc., the proposed optimal energy flow model can maintain the security of IEGS.