Developing sustainable solutions for power generation and transportation is crucial for reducing greenhouse gases (GHGs). The tendency in fundamental and practical research on this field has been characterized by investigating low or net zero-carbon sources. Among the available options, hydrogen may be employed as a possible energy carrier to reduce the local emissions of harmful gases. However, the intensive use of gaseous hydrogen is still limited by the issues related to its storage and transportation systems. In this framework, injection of compressed hydrogen into the existing natural gas pipelines is seen as a way forward to reduce the quantity of carbon-based fuel and will be studied in this work. Compared to building a dedicated hydrogen infrastructure, this solution permits lower capital costs and ensures system scalability (i.e., the possibility to add hydrogen gradually). However, the possible consequences of a gas pipeline failure can be altered. In this paper, a numerical evaluation is conducted to characterize hydrogen-methane mixture jet fire. The analysis is performed using integral models and computational fluid dynamics (CFD). In the case of the computational fluid dynamic approach, an accurate thermodynamic properties database is employed. The distance from the releasing point at which the maximum temperature is reached (i.e., the length of the jet flame) is used as a monitoring parameter for the comparison. Results are compared with existing literature data and discussed to evaluate the safety impact of adding hydrogen to the natural gas network.