Angular2 RC6: '<component> is not a known element'

I received this error when I imported Module A into Module B, and then tried to use a component from Module A in Module B.

The solution is to declare that component in the exports array.

@NgModule({
  declarations: [
    MyComponent
  ],
  exports: [
    MyComponent
  ]
})
export class ModuleA {}

@NgModule({
  imports: [
    ModuleA
  ]
})
export class ModuleB {}

I fixed it with help of Sanket's answer and the comments.

What you couldn't know and was not apparent in the Error Message is: I imported the PlannerComponent as a @NgModule.declaration in my App Module (= RootModule).

The error was fixed by importing the PlannerModule as @NgModule.imports.

Before:

@NgModule({
  declarations: [
    AppComponent,
    PlannerComponent,
    ProfilAreaComponent,
    HeaderAreaComponent,
    NavbarAreaComponent,
    GraphAreaComponent,
    EreignisbarAreaComponent
  ],
  imports: [
    BrowserModule,
    RouterModule.forRoot(routeConfig),
    PlannerModule
  ],
  bootstrap: [AppComponent]
})
export class AppModule {

After:

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    RouterModule.forRoot(routeConfig),
    PlannerModule
  ],
  bootstrap: [AppComponent]
})
export class AppModule {
}

Thanks for your help :)

If you have used the Webclipse automatically generated component definition you may find that the selector name has 'app-' prepended to it. Apparently this is a new convention when declaring sub-components of a main app component. Check how your selector has been defined in your component if you have used 'new' - 'component' to create it in Angular IDE. So instead of putting

<header-area></header-area>

you may need

<app-header-area></app-header-area>

I fetch same problem for <flash-messages></flash-messages> with angular 5.

You just need add below lines in app.module.ts file

import { ---, CUSTOM_ELEMENTS_SCHEMA } from '@angular/core';
import { FlashMessageModule } from "angular-flash-message";


@NgModule({
  ---------------
  imports: [
    FlashMessageModule,
    ------------------         
  ], 
  -----------------
  schemas: [ CUSTOM_ELEMENTS_SCHEMA ]
  ------------
})

NB: I am using this one for message flash-messages