Is DCS part of SCADA system?

No, SCADA supervise and integrate control systems like PLC and DCS, each site has it's own control system like PLC or DCS and SCADA used to connect between these sites and gathering data from all sites. you can monitor and control different location sites from one control room thanks to SCADA.

DCS not part of SCADA DCS needs to be always connected to the I/O of the system, while SCADA is expected to perform even when field communications fail for some time. SCADA does this by keeping a record of all current values

No DCS is NOT a part of SCADA system

, SCADA and DCS were generally detached from each other. However, with constant technological advancement, the two have been blended together making them appear more of same entities. However, as much as these two systems may appear to the same, the solutions and functionality of each, ranging from the goals, the processes' orientation as well as connectivity to the source keeps these two apart differ.