Inspiron 15 3530, heat sink issue?

Unlikely, unless the heatsink has been physically removed and is damaged -- or the system is dusty inside or the thermal compound has dried out.  Is the fan operating correctly?

@ejn63  sorry for late reply and yes it's been removed many times in motherboard replacement and cleaned and I am not confirmed about this that I saw physical dent in heatsink 

@ejn63​ hey please reply 

If so, replace the heat sink.

https://www.parts-people.com/index.php?action=category&id=141&subid=1645&refine=15+heat+sink

@ejn63​ but there is problem that dell says that this is normal and no problem is there for heatsink and I have laptop under warranty 

@ejn63​ hey please reply what to do

It's impossible to advise given the information you've provided.  Other than the temperature differential, which could simply be due to proximity to the CPU, is the system throttling or shutting down due to overheating?

@ejn63​ Yes, the system is thermally throttling heavily. During a GPU stress test, the CPU speed drops to 1.1 GHz and the GPU hits 100% usage with very low FPS.

Most importantly, the fan remains at a low RPM despite the high internal temps, and the exhaust air is barely warm, suggesting the heat pipe is not conducting heat to the vents. If possible please reply Faster.

A GPU stress test isn't a fair evaluation of the system.  They do provide data for workstations and gaming systems - but it's unreasonable to expect a low-end budget notebook to sustain a GPU stress test.  The system was never designed to be a high performer -- it was designed to be low in cost.  You wouldn't run a Hyundai Elantra at redline, and you can't expect the notebook equivalent to sustain the same thing.

Does the system throttle in normal operation with the power management set to a balanced schema?

@ejn63​ Yes, the system throttles during normal operation.

​Even on the Balanced power schema with no heavy apps running, the CPU frequency drops to 1.1 GHz. This causes visible mouse lag and system stuttering during basic web browsing and UI navigation.

​This isn't about asking a 'budget' laptop to do 'workstation' tasks; it's about the laptop failing to maintain its base clock speed for everyday use. The fact that the SupportAssist software itself crashed with a memory error (attached) confirms that the thermal assembly is failing to keep the hardware stable even under low-to-moderate loa ds

@ejn63​ hey please reply its important 

I still don't think there's anything wrong, but you're convinced there is.  Dell won't provide a warranty repair without a demonstrated diagnostic failure.

If you insist there's a problem, buy a new heatsink assembly and replace it or have a local shop do that for you.

@ejn63​ one last question please 

So cold air coming from fan outside while chassis near to cpu is hot normal or not?

If the CPU is not running at high speed or the air in the room is cool, it's normal.