Organisations increasingly need modelling approaches that can support sustainable development decisions across economic, environmental, and social dimensions. The triple-bottom-line (TBL) perspective provides a useful basis for such analysis, but it also introduces modelling challenges because the relevant dimensions are often interdependent, unevenly measurable, and subject to different stakeholder interpretations. Modelling and simulation (M&S) techniques, such as discrete-event simulation (DES), can effectively support sustainability analysis, particularly when TBL considerations are incorporated during the conceptual modelling phase of a simulation study; however, this remains a relatively understudied area in the literature. This paper proposes a framework that positions Qualitative System Dynamics (QSD) as a precursor to DES in TBL-oriented modelling. The framework uses QSD to support conceptual modelling, make system boundaries and assumptions explicit, identify sustainability-relevant key performance indicators, and structure trade-offs before quantitative implementation using DES. The contribution of the paper is conceptual and methodological, clarifying how sustainability considerations can inform model specification rather than being added only at the stage of output interpretation. The framework is illustrated through a retrospective healthcare case study.