Hybrid-electric civil airliners (HECAs) are considered as the forerunner of the solution of relieving aviation emissions. This paper presents a multidisciplinary design analysis and optimisation framework named GENUS, which has been extended to design HECA. GENUS is a modular, expandable, and flexible design environment, with 10 integrated modules for HECA design. Key extensions include hybrid-electric propulsion architectures (HEPAs), the corresponding powertrains and the power management strategies (PMS). In addition, a cost module and an aviation emission tracking function are developed and integrated into GENUS. GENUS is validated for investigating the design of HECAs by evaluating existing HECA concepts. Furthermore, three conventional turbofans are hybridised within GENUS to analyse the sensitivity of the performance of engines to the degree of hybridisation (DoH) of power. The effects of hybridised engines on aircraft design are evaluated based on Boeing 737, demonstrating that at least 27.18% fuel saving, 9.97% energy saving, 12.40% cost saving, and 43.56% aviation emissions migration can be achieved. Finally, the potential directions of applying GENUS to explore the design space of HECA is discussed, which is useful to maximise the benefits of HECA.