Reliable power-delivery network (PDN) design is a major challenge for monolithic 3D (M3D) ICs due to higher power density and current demand per unit area, higher parasitics of interconnects, lower supply voltage, and faster clock. As a first step towards the development of design solutions, we carry out power-supply noise analysis for M3D ICs using electrical and thermal co-simulation. We study the power-supply noise for different PDN designs and analyze design features that impact power-supply noise. This study of power-supply noise motivates further studies on reliable power delivery for M3D ICs.