The unified theory of tubesheet (TS) design for fixed TS heat exchangers (HEX), floating head and U-tube HEX is presented by removing the mid-plane symmetry (MPS) assumption, which assumes a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the HEX or one TS need be considered. All common types of HEXs, with arbitrary combinations of TS configurations, and with arbitrary geometric, material properties and temperature for each component are considered by the unified theory. The effects of unperforated annual plate, TS flange, gravitational (e.g. dead-weight of tubes, catalyst inside tubes, shell side fluid and tube side fluid) and fluid flow pressure loss, bending stiffness of the tubes, TS in-plane stretch, pressure in TS perforations, differential temperature growth in TS radial direction as well as the temperature gradient in TS thickness direction are also evaluated by the unified theory. Theoretical analysis shows that existing theories of TS design can be derived from the unified theory as special deductions. Structure comparison shows the unified theory covers a wide range of applicable HEX configuration well beyond existing theories. Numerical Comparison indicates that predictions given by the unified theory agree well with finite element analysis (FEA), while ASME results are not accurate or not correct.